diff --git a/wp-includes/PHPMailer/PHPMailer.php b/wp-includes/PHPMailer/PHPMailer.php
index 31646b2a84..52e2027859 100644
--- a/wp-includes/PHPMailer/PHPMailer.php
+++ b/wp-includes/PHPMailer/PHPMailer.php
@@ -748,7 +748,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.2.0';
+    const VERSION = '6.3.0';
 
     /**
      * Error severity: message only, continue processing.
@@ -862,18 +862,25 @@ class PHPMailer
             $subject = $this->encodeHeader($this->secureHeader($subject));
         }
         //Calling mail() with null params breaks
+        $this->edebug('Sending with mail()');
+        $this->edebug('Sendmail path: ' . ini_get('sendmail_path'));
+        $this->edebug("Envelope sender: {$this->Sender}");
+        $this->edebug("To: {$to}");
+        $this->edebug("Subject: {$subject}");
+        $this->edebug("Headers: {$header}");
         if (!$this->UseSendmailOptions || null === $params) {
             $result = @mail($to, $subject, $body, $header);
         } else {
+            $this->edebug("Additional params: {$params}");
             $result = @mail($to, $subject, $body, $header, $params);
         }
-
+        $this->edebug('Result: ' . ($result ? 'true' : 'false'));
         return $result;
     }
 
     /**
-     * Output debugging info via user-defined method.
-     * Only generates output if SMTP debug output is enabled (@see SMTP::$do_debug).
+     * Output debugging info via a user-defined method.
+     * Only generates output if debug output is enabled.
      *
      * @see PHPMailer::$Debugoutput
      * @see PHPMailer::$SMTPDebug
@@ -1070,7 +1077,7 @@ class PHPMailer
         $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
         $pos = strrpos($address, '@');
         if (false === $pos) {
-            // At-sign is missing.
+            //At-sign is missing.
             $error_message = sprintf(
                 '%s (%s): %s',
                 $this->lang('invalid_address'),
@@ -1086,7 +1093,7 @@ class PHPMailer
             return false;
         }
         $params = [$kind, $address, $name];
-        // Enqueue addresses with IDN until we know the PHPMailer::$CharSet.
+        //Enqueue addresses with IDN until we know the PHPMailer::$CharSet.
         if (static::idnSupported() && $this->has8bitChars(substr($address, ++$pos))) {
             if ('Reply-To' !== $kind) {
                 if (!array_key_exists($address, $this->RecipientsQueue)) {
@@ -1103,7 +1110,7 @@ class PHPMailer
             return false;
         }
 
-        // Immediately add standard addresses without IDN.
+        //Immediately add standard addresses without IDN.
         return call_user_func_array([$this, 'addAnAddress'], $params);
     }
 
@@ -1191,6 +1198,11 @@ class PHPMailer
                         $address->mailbox . '@' . $address->host
                     )
                 ) {
+                    //Decode the name part if it's present and encoded
+                    if (property_exists($address, 'personal') && preg_match('/^=\?.*\?=$/', $address->personal)) {
+                        $address->personal = mb_decode_mimeheader($address->personal);
+                    }
+
                     $addresses[] = [
                         'name' => (property_exists($address, 'personal') ? $address->personal : ''),
                         'address' => $address->mailbox . '@' . $address->host,
@@ -1214,9 +1226,15 @@ class PHPMailer
                 } else {
                     list($name, $email) = explode('<', $address);
                     $email = trim(str_replace('>', '', $email));
+                    $name = trim($name);
                     if (static::validateAddress($email)) {
+                        //If this name is encoded, decode it
+                        if (preg_match('/^=\?.*\?=$/', $name)) {
+                            $name = mb_decode_mimeheader($name);
+                        }
                         $addresses[] = [
-                            'name' => trim(str_replace(['"', "'"], '', $name)),
+                            //Remove any surrounding quotes and spaces from the name
+                            'name' => trim($name, '\'" '),
                             'address' => $email,
                         ];
                     }
@@ -1242,7 +1260,7 @@ class PHPMailer
     {
         $address = trim($address);
         $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
-        // Don't validate now addresses with IDN. Will be done in send().
+        //Don't validate now addresses with IDN. Will be done in send().
         $pos = strrpos($address, '@');
         if (
             (false === $pos)
@@ -1395,7 +1413,7 @@ class PHPMailer
      */
     public function punyencodeAddress($address)
     {
-        // Verify we have required functions, CharSet, and at-sign.
+        //Verify we have required functions, CharSet, and at-sign.
         $pos = strrpos($address, '@');
         if (
             !empty($this->CharSet) &&
@@ -1403,17 +1421,21 @@ class PHPMailer
             static::idnSupported()
         ) {
             $domain = substr($address, ++$pos);
-            // Verify CharSet string is a valid one, and domain properly encoded in this CharSet.
+            //Verify CharSet string is a valid one, and domain properly encoded in this CharSet.
             if ($this->has8bitChars($domain) && @mb_check_encoding($domain, $this->CharSet)) {
-                $domain = mb_convert_encoding($domain, 'UTF-8', $this->CharSet);
+                //Convert the domain from whatever charset it's in to UTF-8
+                $domain = mb_convert_encoding($domain, self::CHARSET_UTF8, $this->CharSet);
                 //Ignore IDE complaints about this line - method signature changed in PHP 5.4
                 $errorcode = 0;
                 if (defined('INTL_IDNA_VARIANT_UTS46')) {
-                    $punycode = idn_to_ascii($domain, $errorcode, INTL_IDNA_VARIANT_UTS46);
+                    //Use the current punycode standard (appeared in PHP 7.2)
+                    $punycode = idn_to_ascii($domain, $errorcode, \INTL_IDNA_VARIANT_UTS46);
                 } elseif (defined('INTL_IDNA_VARIANT_2003')) {
+                    //Fall back to this old, deprecated/removed encoding
                     // phpcs:ignore PHPCompatibility.Constants.RemovedConstants.intl_idna_variant_2003Deprecated
-                    $punycode = idn_to_ascii($domain, $errorcode, INTL_IDNA_VARIANT_2003);
+                    $punycode = idn_to_ascii($domain, $errorcode, \INTL_IDNA_VARIANT_2003);
                 } else {
+                    //Fall back to a default we don't know about
                     // phpcs:ignore PHPCompatibility.ParameterValues.NewIDNVariantDefault.NotSet
                     $punycode = idn_to_ascii($domain, $errorcode);
                 }
@@ -1464,7 +1486,7 @@ class PHPMailer
     {
         if (
             'smtp' === $this->Mailer
-            || ('mail' === $this->Mailer && (PHP_VERSION_ID >= 80000 || stripos(PHP_OS, 'WIN') === 0))
+            || ('mail' === $this->Mailer && (\PHP_VERSION_ID >= 80000 || stripos(PHP_OS, 'WIN') === 0))
         ) {
             //SMTP mandates RFC-compliant line endings
             //and it's also used with mail() on Windows
@@ -1476,8 +1498,8 @@ class PHPMailer
         //Check for buggy PHP versions that add a header with an incorrect line break
         if (
             'mail' === $this->Mailer
-            && ((PHP_VERSION_ID >= 70000 && PHP_VERSION_ID < 70017)
-                || (PHP_VERSION_ID >= 70100 && PHP_VERSION_ID < 70103))
+            && ((\PHP_VERSION_ID >= 70000 && \PHP_VERSION_ID < 70017)
+                || (\PHP_VERSION_ID >= 70100 && \PHP_VERSION_ID < 70103))
             && ini_get('mail.add_x_header') === '1'
             && stripos(PHP_OS, 'WIN') === 0
         ) {
@@ -1490,10 +1512,10 @@ class PHPMailer
         }
 
         try {
-            $this->error_count = 0; // Reset errors
+            $this->error_count = 0; //Reset errors
             $this->mailHeader = '';
 
-            // Dequeue recipient and Reply-To addresses with IDN
+            //Dequeue recipient and Reply-To addresses with IDN
             foreach (array_merge($this->RecipientsQueue, $this->ReplyToQueue) as $params) {
                 $params[1] = $this->punyencodeAddress($params[1]);
                 call_user_func_array([$this, 'addAnAddress'], $params);
@@ -1502,7 +1524,7 @@ class PHPMailer
                 throw new Exception($this->lang('provide_address'), self::STOP_CRITICAL);
             }
 
-            // Validate From, Sender, and ConfirmReadingTo addresses
+            //Validate From, Sender, and ConfirmReadingTo addresses
             foreach (['From', 'Sender', 'ConfirmReadingTo'] as $address_kind) {
                 $this->$address_kind = trim($this->$address_kind);
                 if (empty($this->$address_kind)) {
@@ -1526,29 +1548,29 @@ class PHPMailer
                 }
             }
 
-            // Set whether the message is multipart/alternative
+            //Set whether the message is multipart/alternative
             if ($this->alternativeExists()) {
                 $this->ContentType = static::CONTENT_TYPE_MULTIPART_ALTERNATIVE;
             }
 
             $this->setMessageType();
-            // Refuse to send an empty message unless we are specifically allowing it
+            //Refuse to send an empty message unless we are specifically allowing it
             if (!$this->AllowEmpty && empty($this->Body)) {
                 throw new Exception($this->lang('empty_message'), self::STOP_CRITICAL);
             }
 
             //Trim subject consistently
             $this->Subject = trim($this->Subject);
-            // Create body before headers in case body makes changes to headers (e.g. altering transfer encoding)
+            //Create body before headers in case body makes changes to headers (e.g. altering transfer encoding)
             $this->MIMEHeader = '';
             $this->MIMEBody = $this->createBody();
-            // createBody may have added some headers, so retain them
+            //createBody may have added some headers, so retain them
             $tempheaders = $this->MIMEHeader;
             $this->MIMEHeader = $this->createHeader();
             $this->MIMEHeader .= $tempheaders;
 
-            // To capture the complete message when using mail(), create
-            // an extra header list which createHeader() doesn't fold in
+            //To capture the complete message when using mail(), create
+            //an extra header list which createHeader() doesn't fold in
             if ('mail' === $this->Mailer) {
                 if (count($this->to) > 0) {
                     $this->mailHeader .= $this->addrAppend('To', $this->to);
@@ -1561,7 +1583,7 @@ class PHPMailer
                 );
             }
 
-            // Sign with DKIM if enabled
+            //Sign with DKIM if enabled
             if (
                 !empty($this->DKIM_domain)
                 && !empty($this->DKIM_selector)
@@ -1602,7 +1624,7 @@ class PHPMailer
     public function postSend()
     {
         try {
-            // Choose the mailer and send through it
+            //Choose the mailer and send through it
             switch ($this->Mailer) {
                 case 'sendmail':
                 case 'qmail':
@@ -1647,22 +1669,45 @@ class PHPMailer
      */
     protected function sendmailSend($header, $body)
     {
+        if ($this->Mailer === 'qmail') {
+            $this->edebug('Sending with qmail');
+        } else {
+            $this->edebug('Sending with sendmail');
+        }
         $header = static::stripTrailingWSP($header) . static::$LE . static::$LE;
-
-        // CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
-        if (!empty($this->Sender) && self::isShellSafe($this->Sender)) {
-            if ('qmail' === $this->Mailer) {
+        //This sets the SMTP envelope sender which gets turned into a return-path header by the receiver
+        //A space after `-f` is optional, but there is a long history of its presence
+        //causing problems, so we don't use one
+        //Exim docs: http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_exim_command_line.html
+        //Sendmail docs: http://www.sendmail.org/~ca/email/man/sendmail.html
+        //Qmail docs: http://www.qmail.org/man/man8/qmail-inject.html
+        //Example problem: https://www.drupal.org/node/1057954
+        //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
+        if ('' === $this->Sender) {
+            $this->Sender = $this->From;
+        }
+        if (empty($this->Sender) && !empty(ini_get('sendmail_from'))) {
+            //PHP config has a sender address we can use
+            $this->Sender = ini_get('sendmail_from');
+        }
+        //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
+        //But sendmail requires this param, so fail without it
+        if (!empty($this->Sender) && static::validateAddress($this->Sender) && self::isShellSafe($this->Sender)) {
+            if ($this->Mailer === 'qmail') {
                 $sendmailFmt = '%s -f%s';
             } else {
                 $sendmailFmt = '%s -oi -f%s -t';
             }
-        } elseif ('qmail' === $this->Mailer) {
-            $sendmailFmt = '%s';
         } else {
-            $sendmailFmt = '%s -oi -t';
+            $this->edebug('Sender address unusable or missing: ' . $this->Sender);
+            return false;
         }
 
         $sendmail = sprintf($sendmailFmt, escapeshellcmd($this->Sendmail), $this->Sender);
+        $this->edebug('Sendmail path: ' . $this->Sendmail);
+        $this->edebug('Sendmail command: ' . $sendmail);
+        $this->edebug('Envelope sender: ' . $this->Sender);
+        $this->edebug("Headers: {$header}");
 
         if ($this->SingleTo) {
             foreach ($this->SingleToArray as $toAddr) {
@@ -1670,6 +1715,7 @@ class PHPMailer
                 if (!$mail) {
                     throw new Exception($this->lang('execute') . $this->Sendmail, self::STOP_CRITICAL);
                 }
+                $this->edebug("To: {$toAddr}");
                 fwrite($mail, 'To: ' . $toAddr . "\n");
                 fwrite($mail, $header);
                 fwrite($mail, $body);
@@ -1684,6 +1730,7 @@ class PHPMailer
                     $this->From,
                     []
                 );
+                $this->edebug("Result: " . ($result === 0 ? 'true' : 'false'));
                 if (0 !== $result) {
                     throw new Exception($this->lang('execute') . $this->Sendmail, self::STOP_CRITICAL);
                 }
@@ -1706,6 +1753,7 @@ class PHPMailer
                 $this->From,
                 []
             );
+            $this->edebug("Result: " . ($result === 0 ? 'true' : 'false'));
             if (0 !== $result) {
                 throw new Exception($this->lang('execute') . $this->Sendmail, self::STOP_CRITICAL);
             }
@@ -1726,7 +1774,7 @@ class PHPMailer
      */
     protected static function isShellSafe($string)
     {
-        // Future-proof
+        //Future-proof
         if (
             escapeshellcmd($string) !== $string
             || !in_array(escapeshellarg($string), ["'$string'", "\"$string\""])
@@ -1739,9 +1787,9 @@ class PHPMailer
         for ($i = 0; $i < $length; ++$i) {
             $c = $string[$i];
 
-            // All other characters have a special meaning in at least one common shell, including = and +.
-            // Full stop (.) has a special meaning in cmd.exe, but its impact should be negligible here.
-            // Note that this does permit non-Latin alphanumeric characters based on the current locale.
+            //All other characters have a special meaning in at least one common shell, including = and +.
+            //Full stop (.) has a special meaning in cmd.exe, but its impact should be negligible here.
+            //Note that this does permit non-Latin alphanumeric characters based on the current locale.
             if (!ctype_alnum($c) && strpos('@_-.', $c) === false) {
                 return false;
             }
@@ -1811,11 +1859,18 @@ class PHPMailer
         //Sendmail docs: http://www.sendmail.org/~ca/email/man/sendmail.html
         //Qmail docs: http://www.qmail.org/man/man8/qmail-inject.html
         //Example problem: https://www.drupal.org/node/1057954
-        // CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
-        if (!empty($this->Sender) && static::validateAddress($this->Sender) && self::isShellSafe($this->Sender)) {
-            $params = sprintf('-f%s', $this->Sender);
+        //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
+        if ('' === $this->Sender) {
+            $this->Sender = $this->From;
+        }
+        if (empty($this->Sender) && !empty(ini_get('sendmail_from'))) {
+            //PHP config has a sender address we can use
+            $this->Sender = ini_get('sendmail_from');
         }
         if (!empty($this->Sender) && static::validateAddress($this->Sender)) {
+            if (self::isShellSafe($this->Sender)) {
+                $params = sprintf('-f%s', $this->Sender);
+            }
             $old_from = ini_get('sendmail_from');
             ini_set('sendmail_from', $this->Sender);
         }
@@ -1901,7 +1956,7 @@ class PHPMailer
         }
 
         $callbacks = [];
-        // Attempt to send to all recipients
+        //Attempt to send to all recipients
         foreach ([$this->to, $this->cc, $this->bcc] as $togroup) {
             foreach ($togroup as $to) {
                 if (!$this->smtp->recipient($to[0], $this->dsn)) {
@@ -1916,7 +1971,7 @@ class PHPMailer
             }
         }
 
-        // Only send the DATA command if we have viable recipients
+        //Only send the DATA command if we have viable recipients
         if ((count($this->all_recipients) > count($bad_rcpt)) && !$this->smtp->data($header . $body)) {
             throw new Exception($this->lang('data_not_accepted'), self::STOP_CRITICAL);
         }
@@ -1978,7 +2033,7 @@ class PHPMailer
             $options = $this->SMTPOptions;
         }
 
-        // Already connected?
+        //Already connected?
         if ($this->smtp->connected()) {
             return true;
         }
@@ -2000,14 +2055,14 @@ class PHPMailer
                 )
             ) {
                 $this->edebug($this->lang('invalid_hostentry') . ' ' . trim($hostentry));
-                // Not a valid host entry
+                //Not a valid host entry
                 continue;
             }
-            // $hostinfo[1]: optional ssl or tls prefix
-            // $hostinfo[2]: the hostname
-            // $hostinfo[3]: optional port number
-            // The host string prefix can temporarily override the current setting for SMTPSecure
-            // If it's not specified, the default value is used
+            //$hostinfo[1]: optional ssl or tls prefix
+            //$hostinfo[2]: the hostname
+            //$hostinfo[3]: optional port number
+            //The host string prefix can temporarily override the current setting for SMTPSecure
+            //If it's not specified, the default value is used
 
             //Check the host name is a valid name or IP address before trying to use it
             if (!static::isValidHost($hostinfo[2])) {
@@ -2019,11 +2074,11 @@ class PHPMailer
             $tls = (static::ENCRYPTION_STARTTLS === $this->SMTPSecure);
             if ('ssl' === $hostinfo[1] || ('' === $hostinfo[1] && static::ENCRYPTION_SMTPS === $this->SMTPSecure)) {
                 $prefix = 'ssl://';
-                $tls = false; // Can't have SSL and TLS at the same time
+                $tls = false; //Can't have SSL and TLS at the same time
                 $secure = static::ENCRYPTION_SMTPS;
             } elseif ('tls' === $hostinfo[1]) {
                 $tls = true;
-                // tls doesn't use a prefix
+                //TLS doesn't use a prefix
                 $secure = static::ENCRYPTION_STARTTLS;
             }
             //Do we need the OpenSSL extension?
@@ -2053,10 +2108,10 @@ class PHPMailer
                     }
                     $this->smtp->hello($hello);
                     //Automatically enable TLS encryption if:
-                    // * it's not disabled
-                    // * we have openssl extension
-                    // * we are not already using SSL
-                    // * the server offers STARTTLS
+                    //* it's not disabled
+                    //* we have openssl extension
+                    //* we are not already using SSL
+                    //* the server offers STARTTLS
                     if ($this->SMTPAutoTLS && $sslext && 'ssl' !== $secure && $this->smtp->getServerExt('STARTTLS')) {
                         $tls = true;
                     }
@@ -2064,7 +2119,7 @@ class PHPMailer
                         if (!$this->smtp->startTLS()) {
                             throw new Exception($this->lang('connect_host'));
                         }
-                        // We must resend EHLO after TLS negotiation
+                        //We must resend EHLO after TLS negotiation
                         $this->smtp->hello($hello);
                     }
                     if (
@@ -2082,14 +2137,14 @@ class PHPMailer
                 } catch (Exception $exc) {
                     $lastexception = $exc;
                     $this->edebug($exc->getMessage());
-                    // We must have connected, but then failed TLS or Auth, so close connection nicely
+                    //We must have connected, but then failed TLS or Auth, so close connection nicely
                     $this->smtp->quit();
                 }
             }
         }
-        // If we get here, all connection attempts have failed, so close connection hard
+        //If we get here, all connection attempts have failed, so close connection hard
         $this->smtp->close();
-        // As we've caught all exceptions, just report whatever the last one was
+        //As we've caught all exceptions, just report whatever the last one was
         if ($this->exceptions && null !== $lastexception) {
             throw $lastexception;
         }
@@ -2120,7 +2175,7 @@ class PHPMailer
      */
     public function setLanguage($langcode = 'en', $lang_path = '')
     {
-        // Backwards compatibility for renamed language codes
+        //Backwards compatibility for renamed language codes
         $renamed_langcodes = [
             'br' => 'pt_br',
             'cz' => 'cs',
@@ -2136,7 +2191,7 @@ class PHPMailer
             $langcode = $renamed_langcodes[$langcode];
         }
 
-        // Define full set of translatable strings in English
+        //Define full set of translatable strings in English
         $PHPMAILER_LANG = [
             'authenticate' => 'SMTP Error: Could not authenticate.',
             'connect_host' => 'SMTP Error: Could not connect to SMTP host.',
@@ -2161,7 +2216,7 @@ class PHPMailer
             'extension_missing' => 'Extension missing: ',
         ];
         if (empty($lang_path)) {
-            // Calculate an absolute path so it can work if CWD is not here
+            //Calculate an absolute path so it can work if CWD is not here
             $lang_path = dirname(__DIR__) . DIRECTORY_SEPARATOR . 'language' . DIRECTORY_SEPARATOR;
         }
         //Validate $langcode
@@ -2170,20 +2225,20 @@ class PHPMailer
         }
         $foundlang = true;
         $lang_file = $lang_path . 'phpmailer.lang-' . $langcode . '.php';
-        // There is no English translation file
+        //There is no English translation file
         if ('en' !== $langcode) {
-            // Make sure language file path is readable
+            //Make sure language file path is readable
             if (!static::fileIsAccessible($lang_file)) {
                 $foundlang = false;
             } else {
-                // Overwrite language-specific strings.
-                // This way we'll never have missing translation keys.
+                //Overwrite language-specific strings.
+                //This way we'll never have missing translation keys.
                 $foundlang = include $lang_file;
             }
         }
         $this->language = $PHPMAILER_LANG;
 
-        return (bool) $foundlang; // Returns false if language not found
+        return (bool) $foundlang; //Returns false if language not found
     }
 
     /**
@@ -2227,7 +2282,7 @@ class PHPMailer
      */
     public function addrFormat($addr)
     {
-        if (empty($addr[1])) { // No name provided
+        if (empty($addr[1])) { //No name provided
             return $this->secureHeader($addr[0]);
         }
 
@@ -2254,8 +2309,8 @@ class PHPMailer
         } else {
             $soft_break = static::$LE;
         }
-        // If utf-8 encoding is used, we will need to make sure we don't
-        // split multibyte characters when we wrap
+        //If utf-8 encoding is used, we will need to make sure we don't
+        //split multibyte characters when we wrap
         $is_utf8 = static::CHARSET_UTF8 === strtolower($this->CharSet);
         $lelen = strlen(static::$LE);
         $crlflen = strlen(static::$LE);
@@ -2355,29 +2410,29 @@ class PHPMailer
             $lastChunk = substr($encodedText, $maxLength - $lookBack, $lookBack);
             $encodedCharPos = strpos($lastChunk, '=');
             if (false !== $encodedCharPos) {
-                // Found start of encoded character byte within $lookBack block.
-                // Check the encoded byte value (the 2 chars after the '=')
+                //Found start of encoded character byte within $lookBack block.
+                //Check the encoded byte value (the 2 chars after the '=')
                 $hex = substr($encodedText, $maxLength - $lookBack + $encodedCharPos + 1, 2);
                 $dec = hexdec($hex);
                 if ($dec < 128) {
-                    // Single byte character.
-                    // If the encoded char was found at pos 0, it will fit
-                    // otherwise reduce maxLength to start of the encoded char
+                    //Single byte character.
+                    //If the encoded char was found at pos 0, it will fit
+                    //otherwise reduce maxLength to start of the encoded char
                     if ($encodedCharPos > 0) {
                         $maxLength -= $lookBack - $encodedCharPos;
                     }
                     $foundSplitPos = true;
                 } elseif ($dec >= 192) {
-                    // First byte of a multi byte character
-                    // Reduce maxLength to split at start of character
+                    //First byte of a multi byte character
+                    //Reduce maxLength to split at start of character
                     $maxLength -= $lookBack - $encodedCharPos;
                     $foundSplitPos = true;
                 } elseif ($dec < 192) {
-                    // Middle byte of a multi byte character, look further back
+                    //Middle byte of a multi byte character, look further back
                     $lookBack += 3;
                 }
             } else {
-                // No encoded character found
+                //No encoded character found
                 $foundSplitPos = true;
             }
         }
@@ -2421,7 +2476,7 @@ class PHPMailer
 
         $result .= $this->headerLine('Date', '' === $this->MessageDate ? self::rfcDate() : $this->MessageDate);
 
-        // The To header is created automatically by mail(), so needs to be omitted here
+        //The To header is created automatically by mail(), so needs to be omitted here
         if ('mail' !== $this->Mailer) {
             if ($this->SingleTo) {
                 foreach ($this->to as $toaddr) {
@@ -2435,12 +2490,12 @@ class PHPMailer
         }
         $result .= $this->addrAppend('From', [[trim($this->From), $this->FromName]]);
 
-        // sendmail and mail() extract Cc from the header before sending
+        //sendmail and mail() extract Cc from the header before sending
         if (count($this->cc) > 0) {
             $result .= $this->addrAppend('Cc', $this->cc);
         }
 
-        // sendmail and mail() extract Bcc from the header before sending
+        //sendmail and mail() extract Bcc from the header before sending
         if (
             (
                 'sendmail' === $this->Mailer || 'qmail' === $this->Mailer || 'mail' === $this->Mailer
@@ -2454,13 +2509,13 @@ class PHPMailer
             $result .= $this->addrAppend('Reply-To', $this->ReplyTo);
         }
 
-        // mail() sets the subject itself
+        //mail() sets the subject itself
         if ('mail' !== $this->Mailer) {
             $result .= $this->headerLine('Subject', $this->encodeHeader($this->secureHeader($this->Subject)));
         }
 
-        // Only allow a custom message ID if it conforms to RFC 5322 section 3.6.4
-        // https://tools.ietf.org/html/rfc5322#section-3.6.4
+        //Only allow a custom message ID if it conforms to RFC 5322 section 3.6.4
+        //https://tools.ietf.org/html/rfc5322#section-3.6.4
         if ('' !== $this->MessageID && preg_match('/^<.*@.*>$/', $this->MessageID)) {
             $this->lastMessageID = $this->MessageID;
         } else {
@@ -2486,7 +2541,7 @@ class PHPMailer
             $result .= $this->headerLine('Disposition-Notification-To', '<' . $this->ConfirmReadingTo . '>');
         }
 
-        // Add custom headers
+        //Add custom headers
         foreach ($this->CustomHeader as $header) {
             $result .= $this->headerLine(
                 trim($header[0]),
@@ -2528,28 +2583,24 @@ class PHPMailer
                 $result .= $this->textLine(' boundary="' . $this->boundary[1] . '"');
                 break;
             default:
-                // Catches case 'plain': and case '':
+                //Catches case 'plain': and case '':
                 $result .= $this->textLine('Content-Type: ' . $this->ContentType . '; charset=' . $this->CharSet);
                 $ismultipart = false;
                 break;
         }
-        // RFC1341 part 5 says 7bit is assumed if not specified
+        //RFC1341 part 5 says 7bit is assumed if not specified
         if (static::ENCODING_7BIT !== $this->Encoding) {
-            // RFC 2045 section 6.4 says multipart MIME parts may only use 7bit, 8bit or binary CTE
+            //RFC 2045 section 6.4 says multipart MIME parts may only use 7bit, 8bit or binary CTE
             if ($ismultipart) {
                 if (static::ENCODING_8BIT === $this->Encoding) {
                     $result .= $this->headerLine('Content-Transfer-Encoding', static::ENCODING_8BIT);
                 }
-                // The only remaining alternatives are quoted-printable and base64, which are both 7bit compatible
+                //The only remaining alternatives are quoted-printable and base64, which are both 7bit compatible
             } else {
                 $result .= $this->headerLine('Content-Transfer-Encoding', $this->Encoding);
             }
         }
 
-        if ('mail' !== $this->Mailer) {
-//            $result .= static::$LE;
-        }
-
         return $result;
     }
 
@@ -2818,7 +2869,7 @@ class PHPMailer
                 $body .= $this->attachAll('attachment', $this->boundary[1]);
                 break;
             default:
-                // Catch case 'plain' and case '', applies to simple `text/plain` and `text/html` body content types
+                //Catch case 'plain' and case '', applies to simple `text/plain` and `text/html` body content types
                 //Reset the `Encoding` property in case we changed it for line length reasons
                 $this->Encoding = $bodyEncoding;
                 $body .= $this->encodeString($this->Body, $this->Encoding);
@@ -2909,7 +2960,7 @@ class PHPMailer
         $result .= $this->textLine('--' . $boundary);
         $result .= sprintf('Content-Type: %s; charset=%s', $contentType, $charSet);
         $result .= static::$LE;
-        // RFC1341 part 5 says 7bit is assumed if not specified
+        //RFC1341 part 5 says 7bit is assumed if not specified
         if (static::ENCODING_7BIT !== $encoding) {
             $result .= $this->headerLine('Content-Transfer-Encoding', $encoding);
         }
@@ -3007,7 +3058,7 @@ class PHPMailer
                 throw new Exception($this->lang('file_access') . $path, self::STOP_CONTINUE);
             }
 
-            // If a MIME type is not specified, try to work it out from the file name
+            //If a MIME type is not specified, try to work it out from the file name
             if ('' === $type) {
                 $type = static::filenameToType($path);
             }
@@ -3026,7 +3077,7 @@ class PHPMailer
                 2 => $name,
                 3 => $encoding,
                 4 => $type,
-                5 => false, // isStringAttachment
+                5 => false, //isStringAttachment
                 6 => $disposition,
                 7 => $name,
             ];
@@ -3066,16 +3117,16 @@ class PHPMailer
      */
     protected function attachAll($disposition_type, $boundary)
     {
-        // Return text of body
+        //Return text of body
         $mime = [];
         $cidUniq = [];
         $incl = [];
 
-        // Add all attachments
+        //Add all attachments
         foreach ($this->attachment as $attachment) {
-            // Check if it is a valid disposition_filter
+            //Check if it is a valid disposition_filter
             if ($attachment[6] === $disposition_type) {
-                // Check for string attachment
+                //Check for string attachment
                 $string = '';
                 $path = '';
                 $bString = $attachment[5];
@@ -3116,7 +3167,7 @@ class PHPMailer
                         static::$LE
                     );
                 }
-                // RFC1341 part 5 says 7bit is assumed if not specified
+                //RFC1341 part 5 says 7bit is assumed if not specified
                 if (static::ENCODING_7BIT !== $encoding) {
                     $mime[] = sprintf('Content-Transfer-Encoding: %s%s', $encoding, static::$LE);
                 }
@@ -3126,7 +3177,7 @@ class PHPMailer
                     $mime[] = 'Content-ID: <' . $this->encodeHeader($this->secureHeader($cid)) . '>' . static::$LE;
                 }
 
-                // Allow for bypassing the Content-Disposition header
+                //Allow for bypassing the Content-Disposition header
                 if (!empty($disposition)) {
                     $encoded_name = $this->encodeHeader($this->secureHeader($name));
                     if (!empty($encoded_name)) {
@@ -3147,7 +3198,7 @@ class PHPMailer
                     $mime[] = static::$LE;
                 }
 
-                // Encode as string attachment
+                //Encode as string attachment
                 if ($bString) {
                     $mime[] = $this->encodeString($string, $encoding);
                 } else {
@@ -3223,7 +3274,7 @@ class PHPMailer
             case static::ENCODING_7BIT:
             case static::ENCODING_8BIT:
                 $encoded = static::normalizeBreaks($str);
-                // Make sure it ends with a line break
+                //Make sure it ends with a line break
                 if (substr($encoded, -(strlen(static::$LE))) !== static::$LE) {
                     $encoded .= static::$LE;
                 }
@@ -3261,7 +3312,7 @@ class PHPMailer
         switch (strtolower($position)) {
             case 'phrase':
                 if (!preg_match('/[\200-\377]/', $str)) {
-                    // Can't use addslashes as we don't know the value of magic_quotes_sybase
+                    //Can't use addslashes as we don't know the value of magic_quotes_sybase
                     $encoded = addcslashes($str, "\0..\37\177\\\"");
                     if (($str === $encoded) && !preg_match('/[^A-Za-z0-9!#$%&\'*+\/=?^_`{|}~ -]/', $str)) {
                         return $encoded;
@@ -3287,7 +3338,7 @@ class PHPMailer
             $charset = static::CHARSET_ASCII;
         }
 
-        // Q/B encoding adds 8 chars and the charset ("` =?<charset>?[QB]?<content>?=`").
+        //Q/B encoding adds 8 chars and the charset ("` =?<charset>?[QB]?<content>?=`").
         $overhead = 8 + strlen($charset);
 
         if ('mail' === $this->Mailer) {
@@ -3296,26 +3347,26 @@ class PHPMailer
             $maxlen = static::MAX_LINE_LENGTH - $overhead;
         }
 
-        // Select the encoding that produces the shortest output and/or prevents corruption.
+        //Select the encoding that produces the shortest output and/or prevents corruption.
         if ($matchcount > strlen($str) / 3) {
-            // More than 1/3 of the content needs encoding, use B-encode.
+            //More than 1/3 of the content needs encoding, use B-encode.
             $encoding = 'B';
         } elseif ($matchcount > 0) {
-            // Less than 1/3 of the content needs encoding, use Q-encode.
+            //Less than 1/3 of the content needs encoding, use Q-encode.
             $encoding = 'Q';
         } elseif (strlen($str) > $maxlen) {
-            // No encoding needed, but value exceeds max line length, use Q-encode to prevent corruption.
+            //No encoding needed, but value exceeds max line length, use Q-encode to prevent corruption.
             $encoding = 'Q';
         } else {
-            // No reformatting needed
+            //No reformatting needed
             $encoding = false;
         }
 
         switch ($encoding) {
             case 'B':
                 if ($this->hasMultiBytes($str)) {
-                    // Use a custom function which correctly encodes and wraps long
-                    // multibyte strings without breaking lines within a character
+                    //Use a custom function which correctly encodes and wraps long
+                    //multibyte strings without breaking lines within a character
                     $encoded = $this->base64EncodeWrapMB($str, "\n");
                 } else {
                     $encoded = base64_encode($str);
@@ -3350,7 +3401,7 @@ class PHPMailer
             return strlen($str) > mb_strlen($str, $this->CharSet);
         }
 
-        // Assume no multibytes (we can't handle without mbstring functions anyway)
+        //Assume no multibytes (we can't handle without mbstring functions anyway)
         return false;
     }
 
@@ -3388,11 +3439,11 @@ class PHPMailer
         }
 
         $mb_length = mb_strlen($str, $this->CharSet);
-        // Each line must have length <= 75, including $start and $end
+        //Each line must have length <= 75, including $start and $end
         $length = 75 - strlen($start) - strlen($end);
-        // Average multi-byte ratio
+        //Average multi-byte ratio
         $ratio = $mb_length / strlen($str);
-        // Base64 has a 4:3 ratio
+        //Base64 has a 4:3 ratio
         $avgLength = floor($length * $ratio * .75);
 
         $offset = 0;
@@ -3407,7 +3458,7 @@ class PHPMailer
             $encoded .= $chunk . $linebreak;
         }
 
-        // Chomp the last linefeed
+        //Chomp the last linefeed
         return substr($encoded, 0, -strlen($linebreak));
     }
 
@@ -3436,12 +3487,12 @@ class PHPMailer
      */
     public function encodeQ($str, $position = 'text')
     {
-        // There should not be any EOL in the string
+        //There should not be any EOL in the string
         $pattern = '';
         $encoded = str_replace(["\r", "\n"], '', $str);
         switch (strtolower($position)) {
             case 'phrase':
-                // RFC 2047 section 5.3
+                //RFC 2047 section 5.3
                 $pattern = '^A-Za-z0-9!*+\/ -';
                 break;
             /*
@@ -3454,15 +3505,15 @@ class PHPMailer
             /* Intentional fall through */
             case 'text':
             default:
-                // RFC 2047 section 5.1
-                // Replace every high ascii, control, =, ? and _ characters
+                //RFC 2047 section 5.1
+                //Replace every high ascii, control, =, ? and _ characters
                 $pattern = '\000-\011\013\014\016-\037\075\077\137\177-\377' . $pattern;
                 break;
         }
         $matches = [];
         if (preg_match_all("/[{$pattern}]/", $encoded, $matches)) {
-            // If the string contains an '=', make sure it's the first thing we replace
-            // so as to avoid double-encoding
+            //If the string contains an '=', make sure it's the first thing we replace
+            //so as to avoid double-encoding
             $eqkey = array_search('=', $matches[0], true);
             if (false !== $eqkey) {
                 unset($matches[0][$eqkey]);
@@ -3472,8 +3523,8 @@ class PHPMailer
                 $encoded = str_replace($char, '=' . sprintf('%02X', ord($char)), $encoded);
             }
         }
-        // Replace spaces with _ (more readable than =20)
-        // RFC 2047 section 4.2(2)
+        //Replace spaces with _ (more readable than =20)
+        //RFC 2047 section 4.2(2)
         return str_replace(' ', '_', $encoded);
     }
 
@@ -3500,7 +3551,7 @@ class PHPMailer
         $disposition = 'attachment'
     ) {
         try {
-            // If a MIME type is not specified, try to work it out from the file name
+            //If a MIME type is not specified, try to work it out from the file name
             if ('' === $type) {
                 $type = static::filenameToType($filename);
             }
@@ -3509,14 +3560,14 @@ class PHPMailer
                 throw new Exception($this->lang('encoding') . $encoding);
             }
 
-            // Append to $attachment array
+            //Append to $attachment array
             $this->attachment[] = [
                 0 => $string,
                 1 => $filename,
                 2 => static::mb_pathinfo($filename, PATHINFO_BASENAME),
                 3 => $encoding,
                 4 => $type,
-                5 => true, // isStringAttachment
+                5 => true, //isStringAttachment
                 6 => $disposition,
                 7 => 0,
             ];
@@ -3567,7 +3618,7 @@ class PHPMailer
                 throw new Exception($this->lang('file_access') . $path, self::STOP_CONTINUE);
             }
 
-            // If a MIME type is not specified, try to work it out from the file name
+            //If a MIME type is not specified, try to work it out from the file name
             if ('' === $type) {
                 $type = static::filenameToType($path);
             }
@@ -3581,14 +3632,14 @@ class PHPMailer
                 $name = $filename;
             }
 
-            // Append to $attachment array
+            //Append to $attachment array
             $this->attachment[] = [
                 0 => $path,
                 1 => $filename,
                 2 => $name,
                 3 => $encoding,
                 4 => $type,
-                5 => false, // isStringAttachment
+                5 => false, //isStringAttachment
                 6 => $disposition,
                 7 => $cid,
             ];
@@ -3633,7 +3684,7 @@ class PHPMailer
         $disposition = 'inline'
     ) {
         try {
-            // If a MIME type is not specified, try to work it out from the name
+            //If a MIME type is not specified, try to work it out from the name
             if ('' === $type && !empty($name)) {
                 $type = static::filenameToType($name);
             }
@@ -3642,14 +3693,14 @@ class PHPMailer
                 throw new Exception($this->lang('encoding') . $encoding);
             }
 
-            // Append to $attachment array
+            //Append to $attachment array
             $this->attachment[] = [
                 0 => $string,
                 1 => $name,
                 2 => $name,
                 3 => $encoding,
                 4 => $type,
-                5 => true, // isStringAttachment
+                5 => true, //isStringAttachment
                 6 => $disposition,
                 7 => $cid,
             ];
@@ -3869,8 +3920,8 @@ class PHPMailer
      */
     public static function rfcDate()
     {
-        // Set the time zone to whatever the default is to avoid 500 errors
-        // Will default to UTC if it's not set properly in php.ini
+        //Set the time zone to whatever the default is to avoid 500 errors
+        //Will default to UTC if it's not set properly in php.ini
         date_default_timezone_set(@date_default_timezone_get());
 
         return date('D, j M Y H:i:s O');
@@ -3948,13 +3999,13 @@ class PHPMailer
     protected function lang($key)
     {
         if (count($this->language) < 1) {
-            $this->setLanguage(); // set the default language
+            $this->setLanguage(); //Set the default language
         }
 
         if (array_key_exists($key, $this->language)) {
             if ('smtp_connect_failed' === $key) {
-                //Include a link to troubleshooting docs on SMTP connection failure
-                //this is by far the biggest cause of support questions
+                //Include a link to troubleshooting docs on SMTP connection failure.
+                //This is by far the biggest cause of support questions
                 //but it's usually not PHPMailer's fault.
                 return $this->language[$key] . ' https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting';
             }
@@ -3989,7 +4040,7 @@ class PHPMailer
     public function addCustomHeader($name, $value = null)
     {
         if (null === $value && strpos($name, ':') !== false) {
-            // Value passed in as name:value
+            //Value passed in as name:value
             list($name, $value) = explode(':', $name, 2);
         }
         $name = trim($name);
@@ -4043,11 +4094,11 @@ class PHPMailer
         preg_match_all('/(?<!-)(src|background)=["\'](.*)["\']/Ui', $message, $images);
         if (array_key_exists(2, $images)) {
             if (strlen($basedir) > 1 && '/' !== substr($basedir, -1)) {
-                // Ensure $basedir has a trailing /
+                //Ensure $basedir has a trailing /
                 $basedir .= '/';
             }
             foreach ($images[2] as $imgindex => $url) {
-                // Convert data URIs into embedded images
+                //Convert data URIs into embedded images
                 //e.g. "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
                 $match = [];
                 if (preg_match('#^data:(image/(?:jpe?g|gif|png));?(base64)?,(.+)#', $url, $match)) {
@@ -4061,7 +4112,7 @@ class PHPMailer
                     }
                     //Hash the decoded data, not the URL, so that the same data-URI image used in multiple places
                     //will only be embedded once, even if it used a different encoding
-                    $cid = substr(hash('sha256', $data), 0, 32) . '@phpmailer.0'; // RFC2392 S 2
+                    $cid = substr(hash('sha256', $data), 0, 32) . '@phpmailer.0'; //RFC2392 S 2
 
                     if (!$this->cidExists($cid)) {
                         $this->addStringEmbeddedImage(
@@ -4080,13 +4131,13 @@ class PHPMailer
                     continue;
                 }
                 if (
-                    // Only process relative URLs if a basedir is provided (i.e. no absolute local paths)
+                    //Only process relative URLs if a basedir is provided (i.e. no absolute local paths)
                     !empty($basedir)
-                    // Ignore URLs containing parent dir traversal (..)
+                    //Ignore URLs containing parent dir traversal (..)
                     && (strpos($url, '..') === false)
-                    // Do not change urls that are already inline images
+                    //Do not change urls that are already inline images
                     && 0 !== strpos($url, 'cid:')
-                    // Do not change absolute URLs, including anonymous protocol
+                    //Do not change absolute URLs, including anonymous protocol
                     && !preg_match('#^[a-z][a-z0-9+.-]*:?//#i', $url)
                 ) {
                     $filename = static::mb_pathinfo($url, PATHINFO_BASENAME);
@@ -4094,7 +4145,7 @@ class PHPMailer
                     if ('.' === $directory) {
                         $directory = '';
                     }
-                    // RFC2392 S 2
+                    //RFC2392 S 2
                     $cid = substr(hash('sha256', $url), 0, 32) . '@phpmailer.0';
                     if (strlen($basedir) > 1 && '/' !== substr($basedir, -1)) {
                         $basedir .= '/';
@@ -4121,7 +4172,7 @@ class PHPMailer
             }
         }
         $this->isHTML();
-        // Convert all message body line breaks to LE, makes quoted-printable encoding work much better
+        //Convert all message body line breaks to LE, makes quoted-printable encoding work much better
         $this->Body = static::normalizeBreaks($message);
         $this->AltBody = static::normalizeBreaks($this->html2text($message, $advanced));
         if (!$this->alternativeExists()) {
@@ -4140,9 +4191,9 @@ class PHPMailer
      * Example usage:
      *
      * ```php
-     * // Use default conversion
+     * //Use default conversion
      * $plain = $mail->html2text($html);
-     * // Use your own custom converter
+     * //Use your own custom converter
      * $plain = $mail->html2text($html, function($html) {
      *     $converter = new MyHtml2text($html);
      *     return $converter->get_text();
@@ -4309,7 +4360,7 @@ class PHPMailer
      */
     public static function filenameToType($filename)
     {
-        // In case the path is a URL, strip any query string before getting extension
+        //In case the path is a URL, strip any query string before getting extension
         $qpos = strpos($filename, '?');
         if (false !== $qpos) {
             $filename = substr($filename, 0, $qpos);
@@ -4420,9 +4471,9 @@ class PHPMailer
         if (null === $breaktype) {
             $breaktype = static::$LE;
         }
-        // Normalise to \n
+        //Normalise to \n
         $text = str_replace([self::CRLF, "\r"], "\n", $text);
-        // Now convert LE as needed
+        //Now convert LE as needed
         if ("\n" !== $breaktype) {
             $text = str_replace("\n", $breaktype, $text);
         }
@@ -4528,13 +4579,13 @@ class PHPMailer
             $privKey = openssl_pkey_get_private($privKeyStr);
         }
         if (openssl_sign($signHeader, $signature, $privKey, 'sha256WithRSAEncryption')) {
-            if (PHP_MAJOR_VERSION < 8) {
+            if (\PHP_MAJOR_VERSION < 8) {
                 openssl_pkey_free($privKey);
             }
 
             return base64_encode($signature);
         }
-        if (PHP_MAJOR_VERSION < 8) {
+        if (\PHP_MAJOR_VERSION < 8) {
             openssl_pkey_free($privKey);
         }
 
@@ -4601,7 +4652,7 @@ class PHPMailer
         if (empty($body)) {
             return self::CRLF;
         }
-        // Normalize line endings to CRLF
+        //Normalize line endings to CRLF
         $body = static::normalizeBreaks($body, self::CRLF);
 
         //Reduce multiple trailing line breaks to a single one
@@ -4621,9 +4672,9 @@ class PHPMailer
      */
     public function DKIM_Add($headers_line, $subject, $body)
     {
-        $DKIMsignatureType = 'rsa-sha256'; // Signature & hash algorithms
-        $DKIMcanonicalization = 'relaxed/simple'; // Canonicalization methods of header & body
-        $DKIMquery = 'dns/txt'; // Query method
+        $DKIMsignatureType = 'rsa-sha256'; //Signature & hash algorithms
+        $DKIMcanonicalization = 'relaxed/simple'; //Canonicalization methods of header & body
+        $DKIMquery = 'dns/txt'; //Query method
         $DKIMtime = time();
         //Always sign these headers without being asked
         //Recommended list from https://tools.ietf.org/html/rfc6376#section-5.4.1
@@ -4724,7 +4775,8 @@ class PHPMailer
         $headerKeys = ' h=' . implode(':', $headersToSignKeys) . ';' . static::$LE;
         $headerValues = implode(static::$LE, $headersToSign);
         $body = $this->DKIM_BodyC($body);
-        $DKIMb64 = base64_encode(pack('H*', hash('sha256', $body))); // Base64 of packed binary SHA-256 hash of body
+        //Base64 of packed binary SHA-256 hash of body
+        $DKIMb64 = base64_encode(pack('H*', hash('sha256', $body)));
         $ident = '';
         if ('' !== $this->DKIM_identity) {
             $ident = ' i=' . $this->DKIM_identity . ';' . static::$LE;
diff --git a/wp-includes/PHPMailer/SMTP.php b/wp-includes/PHPMailer/SMTP.php
index ab7f46e4c8..68f3aeccc5 100644
--- a/wp-includes/PHPMailer/SMTP.php
+++ b/wp-includes/PHPMailer/SMTP.php
@@ -35,7 +35,7 @@ class SMTP
      *
      * @var string
      */
-    const VERSION = '6.2.0';
+    const VERSION = '6.3.0';
 
     /**
      * SMTP line break constant.
@@ -312,11 +312,11 @@ class SMTP
      */
     public function connect($host, $port = null, $timeout = 30, $options = [])
     {
-        // Clear errors to avoid confusion
+        //Clear errors to avoid confusion
         $this->setError('');
-        // Make sure we are __not__ connected
+        //Make sure we are __not__ connected
         if ($this->connected()) {
-            // Already connected, generate error
+            //Already connected, generate error
             $this->setError('Already connected to a server');
 
             return false;
@@ -324,7 +324,7 @@ class SMTP
         if (empty($port)) {
             $port = self::DEFAULT_PORT;
         }
-        // Connect to the SMTP server
+        //Connect to the SMTP server
         $this->edebug(
             "Connection: opening to $host:$port, timeout=$timeout, options=" .
             (count($options) > 0 ? var_export($options, true) : 'array()'),
@@ -340,11 +340,23 @@ class SMTP
 
         $this->edebug('Connection: opened', self::DEBUG_CONNECTION);
 
-        // Get any announcement
+        //Get any announcement
         $this->last_reply = $this->get_lines();
         $this->edebug('SERVER -> CLIENT: ' . $this->last_reply, self::DEBUG_SERVER);
-
-        return true;
+        $responseCode = (int)substr($this->last_reply, 0, 3);
+        if ($responseCode === 220) {
+            return true;
+        }
+        //Anything other than a 220 response means something went wrong
+        //RFC 5321 says the server will wait for us to send a QUIT in response to a 554 error
+        //https://tools.ietf.org/html/rfc5321#section-3.1
+        if ($responseCode === 554) {
+            $this->quit();
+        }
+        //This will handle 421 responses which may not wait for a QUIT (e.g. if the server is being shut down)
+        $this->edebug('Connection: closing due to error', self::DEBUG_CONNECTION);
+        $this->close();
+        return false;
     }
 
     /**
@@ -397,7 +409,7 @@ class SMTP
             restore_error_handler();
         }
 
-        // Verify we connected properly
+        //Verify we connected properly
         if (!is_resource($connection)) {
             $this->setError(
                 'Failed to connect to server',
@@ -414,11 +426,11 @@ class SMTP
             return false;
         }
 
-        // SMTP server can take longer to respond, give longer timeout for first read
-        // Windows does not have support for this timeout function
+        //SMTP server can take longer to respond, give longer timeout for first read
+        //Windows does not have support for this timeout function
         if (strpos(PHP_OS, 'WIN') !== 0) {
             $max = (int)ini_get('max_execution_time');
-            // Don't bother if unlimited, or if set_time_limit is disabled
+            //Don't bother if unlimited, or if set_time_limit is disabled
             if (0 !== $max && $timeout > $max && strpos(ini_get('disable_functions'), 'set_time_limit') === false) {
                 @set_time_limit($timeout);
             }
@@ -449,7 +461,7 @@ class SMTP
             $crypto_method |= STREAM_CRYPTO_METHOD_TLSv1_1_CLIENT;
         }
 
-        // Begin encrypted connection
+        //Begin encrypted connection
         set_error_handler([$this, 'errorHandler']);
         $crypto_ok = stream_socket_enable_crypto(
             $this->smtp_conn,
@@ -487,11 +499,11 @@ class SMTP
         }
 
         if (array_key_exists('EHLO', $this->server_caps)) {
-            // SMTP extensions are available; try to find a proper authentication method
+            //SMTP extensions are available; try to find a proper authentication method
             if (!array_key_exists('AUTH', $this->server_caps)) {
                 $this->setError('Authentication is not allowed at this stage');
-                // 'at this stage' means that auth may be allowed after the stage changes
-                // e.g. after STARTTLS
+                //'at this stage' means that auth may be allowed after the stage changes
+                //e.g. after STARTTLS
 
                 return false;
             }
@@ -535,11 +547,11 @@ class SMTP
         }
         switch ($authtype) {
             case 'PLAIN':
-                // Start authentication
+                //Start authentication
                 if (!$this->sendCommand('AUTH', 'AUTH PLAIN', 334)) {
                     return false;
                 }
-                // Send encoded username and password
+                //Send encoded username and password
                 if (
                     !$this->sendCommand(
                         'User & Password',
@@ -551,7 +563,7 @@ class SMTP
                 }
                 break;
             case 'LOGIN':
-                // Start authentication
+                //Start authentication
                 if (!$this->sendCommand('AUTH', 'AUTH LOGIN', 334)) {
                     return false;
                 }
@@ -563,17 +575,17 @@ class SMTP
                 }
                 break;
             case 'CRAM-MD5':
-                // Start authentication
+                //Start authentication
                 if (!$this->sendCommand('AUTH CRAM-MD5', 'AUTH CRAM-MD5', 334)) {
                     return false;
                 }
-                // Get the challenge
+                //Get the challenge
                 $challenge = base64_decode(substr($this->last_reply, 4));
 
-                // Build the response
+                //Build the response
                 $response = $username . ' ' . $this->hmac($challenge, $password);
 
-                // send encoded credentials
+                //send encoded credentials
                 return $this->sendCommand('Username', base64_encode($response), 235);
             case 'XOAUTH2':
                 //The OAuth instance must be set up prior to requesting auth.
@@ -582,7 +594,7 @@ class SMTP
                 }
                 $oauth = $OAuth->getOauth64();
 
-                // Start authentication
+                //Start authentication
                 if (!$this->sendCommand('AUTH', 'AUTH XOAUTH2 ' . $oauth, 235)) {
                     return false;
                 }
@@ -612,15 +624,15 @@ class SMTP
             return hash_hmac('md5', $data, $key);
         }
 
-        // The following borrowed from
-        // http://php.net/manual/en/function.mhash.php#27225
+        //The following borrowed from
+        //http://php.net/manual/en/function.mhash.php#27225
 
-        // RFC 2104 HMAC implementation for php.
-        // Creates an md5 HMAC.
-        // Eliminates the need to install mhash to compute a HMAC
-        // by Lance Rushing
+        //RFC 2104 HMAC implementation for php.
+        //Creates an md5 HMAC.
+        //Eliminates the need to install mhash to compute a HMAC
+        //by Lance Rushing
 
-        $bytelen = 64; // byte length for md5
+        $bytelen = 64; //byte length for md5
         if (strlen($key) > $bytelen) {
             $key = pack('H*', md5($key));
         }
@@ -643,7 +655,7 @@ class SMTP
         if (is_resource($this->smtp_conn)) {
             $sock_status = stream_get_meta_data($this->smtp_conn);
             if ($sock_status['eof']) {
-                // The socket is valid but we are not connected
+                //The socket is valid but we are not connected
                 $this->edebug(
                     'SMTP NOTICE: EOF caught while checking if connected',
                     self::DEBUG_CLIENT
@@ -653,7 +665,7 @@ class SMTP
                 return false;
             }
 
-            return true; // everything looks good
+            return true; //everything looks good
         }
 
         return false;
@@ -671,7 +683,7 @@ class SMTP
         $this->server_caps = null;
         $this->helo_rply = null;
         if (is_resource($this->smtp_conn)) {
-            // close the connection and cleanup
+            //Close the connection and cleanup
             fclose($this->smtp_conn);
             $this->smtp_conn = null; //Makes for cleaner serialization
             $this->edebug('Connection: closed', self::DEBUG_CONNECTION);
@@ -706,7 +718,7 @@ class SMTP
          * NOTE: this does not count towards line-length limit.
          */
 
-        // Normalize line breaks before exploding
+        //Normalize line breaks before exploding
         $lines = explode("\n", str_replace(["\r\n", "\r"], "\n", $msg_data));
 
         /* To distinguish between a complete RFC822 message and a plain message body, we check if the first field
@@ -752,7 +764,8 @@ class SMTP
 
             //Send the lines to the server
             foreach ($lines_out as $line_out) {
-                //RFC2821 section 4.5.2
+                //Dot-stuffing as per RFC5321 section 4.5.2
+                //https://tools.ietf.org/html/rfc5321#section-4.5.2
                 if (!empty($line_out) && $line_out[0] === '.') {
                     $line_out = '.' . $line_out;
                 }
@@ -786,7 +799,16 @@ class SMTP
     public function hello($host = '')
     {
         //Try extended hello first (RFC 2821)
-        return $this->sendHello('EHLO', $host) or $this->sendHello('HELO', $host);
+        if ($this->sendHello('EHLO', $host)) {
+            return true;
+        }
+
+        //Some servers shut down the SMTP service here (RFC 5321)
+        if (substr($this->helo_rply, 0, 3) == '421') {
+            return false;
+        }
+
+        return $this->sendHello('HELO', $host);
     }
 
     /**
@@ -976,12 +998,12 @@ class SMTP
         $this->client_send($commandstring . static::LE, $command);
 
         $this->last_reply = $this->get_lines();
-        // Fetch SMTP code and possible error code explanation
+        //Fetch SMTP code and possible error code explanation
         $matches = [];
         if (preg_match('/^([\d]{3})[ -](?:([\d]\\.[\d]\\.[\d]{1,2}) )?/', $this->last_reply, $matches)) {
             $code = (int) $matches[1];
             $code_ex = (count($matches) > 2 ? $matches[2] : null);
-            // Cut off error code from each response line
+            //Cut off error code from each response line
             $detail = preg_replace(
                 "/{$code}[ -]" .
                 ($code_ex ? str_replace('.', '\\.', $code_ex) . ' ' : '') . '/m',
@@ -989,7 +1011,7 @@ class SMTP
                 $this->last_reply
             );
         } else {
-            // Fall back to simple parsing if regex fails
+            //Fall back to simple parsing if regex fails
             $code = (int) substr($this->last_reply, 0, 3);
             $code_ex = null;
             $detail = substr($this->last_reply, 4);
@@ -1184,7 +1206,7 @@ class SMTP
      */
     protected function get_lines()
     {
-        // If the connection is bad, give up straight away
+        //If the connection is bad, give up straight away
         if (!is_resource($this->smtp_conn)) {
             return '';
         }
@@ -1237,13 +1259,13 @@ class SMTP
             $str = @fgets($this->smtp_conn, self::MAX_REPLY_LENGTH);
             $this->edebug('SMTP INBOUND: "' . trim($str) . '"', self::DEBUG_LOWLEVEL);
             $data .= $str;
-            // If response is only 3 chars (not valid, but RFC5321 S4.2 says it must be handled),
-            // or 4th character is a space or a line break char, we are done reading, break the loop.
-            // String array access is a significant micro-optimisation over strlen
+            //If response is only 3 chars (not valid, but RFC5321 S4.2 says it must be handled),
+            //or 4th character is a space or a line break char, we are done reading, break the loop.
+            //String array access is a significant micro-optimisation over strlen
             if (!isset($str[3]) || $str[3] === ' ' || $str[3] === "\r" || $str[3] === "\n") {
                 break;
             }
-            // Timed-out? Log and break
+            //Timed-out? Log and break
             $info = stream_get_meta_data($this->smtp_conn);
             if ($info['timed_out']) {
                 $this->edebug(
@@ -1252,7 +1274,7 @@ class SMTP
                 );
                 break;
             }
-            // Now check if reads took too long
+            //Now check if reads took too long
             if ($endtime && time() > $endtime) {
                 $this->edebug(
                     'SMTP -> get_lines(): timelimit reached (' .
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 1b847b7854..53bc05faaf 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-beta3-50396';
+$wp_version = '5.7-beta3-50397';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.