BUGFIX: catch STMP client errors in email sending background job

Will log the exception message in the skipped reason instead of keeping
the job in the queue whenever the email address is invalid

lib/email/sender.rb

@@ -9,6 +9,8 @@
 require_dependency 'email/renderer'
 require 'uri'
 
+SMTP_CLIENT_ERRORS = [Net::SMTPFatalError, Net::SMTPSyntaxError]
+
 module Email
   class Sender
 
@@ -77,12 +79,15 @@ module Email
       @message.header['X-Discourse-Post-Id'] = nil
       @message.header['X-Discourse-Reply-Key'] = nil
 
-      @message.deliver
+      begin
+        @message.deliver
+      rescue SMTP_CLIENT_ERRORS => e
+        return skip(e.message)
+      end
 
       # Save and return the email log
       email_log.save!
       email_log
-
     end
 
     def to_address