Mail: Disable wp-mail.php when `mailserver_url` is mail.example.com.

Merges [39772] to the 4.5 branch.

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git-svn-id: http://core.svn.wordpress.org/branches/4.5@39713 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-mail.php

@@ -14,6 +14,12 @@ require(dirname(__FILE__) . '/wp-load.php');
 if ( ! apply_filters( 'enable_post_by_email_configuration', true ) )
 	wp_die( __( 'This action has been disabled by the administrator.' ) );
 
+$mailserver_url = get_option( 'mailserver_url' );
+
+if ( 'mail.example.com' === $mailserver_url || empty( $mailserver_url ) ) {
+	wp_die( __( 'This action has been disabled by the administrator.' ), 403 );
+}
+
 /**
  * Fires to allow a plugin to do a complete takeover of Post by Email.
  *