XF 2.2 Bad handling of error produced from mail addresses that do not exist / bad domain - cPanel smtp/imap


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The default mail system on all cPanel servers is swiftmailer. When an email is sent through this swiftmailer system the first thing is does is check if the domain exists before it does anything else. If this check fails it reports back a "503" error that the service is unavailable and never even attempts to send the already doomed email.

Currently XenForo does not handle this error well because it appears it is looking for a "354" error when emailing.

It is not unusual for a user when signing up for a forum account to mistype or even intentionally input a bad email which can very often contain a bad domain, so this issue comes up often and a single sign-up can cause this...


Users signs up 2 errors immediately, 5 minutes later another 2 errors, 1 hour later another 2 errors, 2 hours later another 2 errors, 6 hours later another 2 errors. 10 errors for a single bad email address that cPanel/swiftmailer identifies as a "503" service unavailable / bad domain every time.

XenForo has initially said this is a "server error" (https://xenforo.com/community/threads/user-emails-that-were-deleted-still-cause-errors-in-acp.192308). cPanel/swiftmailer have responded saying this is working and responding as designed.

There has to be a solution to suppress the first "503" error and abandon any further mail attempts to addresses with identified bad domains. The problem is I have no clue how to go about it.

This issue should be affecting all cPanel 7 (version 94+) admins using smtp/imap mailing. Which I believe is likely a decent number of admins/forums.

Looking for some help/direction, thanks.
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