5.7.1 Postfix Mail Error

We have had a client report an intermittent mail issue where their email was being bounced with a 5.7.1 error. We host around 10 email accounts for this client, but only act as a pass-through POP - as email is redirected to Gmail servers.

The reported issue was that every few months or so, three of the ten email addresses would start bouncing email back from our server with a 5.7.1 error. Post fix mail logs weren't very helpful since 5.7.1 is a pretty generic error code related to 'no mailbox' or 'mailbox quota exceeded' issues. We knew that neither of these were the issue since we didn't actually store any email, we merely redirected it. Besides, it was simultaneously affecting three different email addresses at the same time. Stranger still, if we deleted the mail accounts and re-created them, they would happily start forwarding email again for another few months before seemingly randomly starting to bounce messages again.

So what weird and wonderful configuration issue could corrupt the same three email accounts at seemingly random periods and force them to start bouncing email? Turns out to be very mundane as most of these things do. In Plesk 9.5.4 (and probably other versions), just setting up a redirect doesn't stop email being stored locally, you have to physically disable the mailbox attached to the email address. Don't worry, the email still gets processed and redirected by Postfix without a mailbox, it just can't store a local copy of the message.

What was happening is that the Postfix was happily redirecting email on to the intended recipient but also saving a copy in the local mailbox until it reached an arbitrary limit of 10Mb that we had set as a default on that domain. It would then start bouncing email back with a 5.7.1 error. By deleting and recreating the email accounts, we were of course deleting the over-quota mailboxes, thereby bringing them back within quota. The other red herring was the coincidence of three email addresses being affected at the same time, but it was just that, a coincidence. The three affected mailboxes were growing and reaching quota at about the same time - possibly because they were all being copied in on the same correspondence.

It would have been extremely helpful if Postfix had reported the usual 5.0.0 Mail Quota exceeded error. I wonder how many other redirected Plesk mailboxes there are out there that are ballooning out to huge proportions because people 'know' their email is being redirected to another mailbox - particularly if there is no quota set.!?

I hope this blog helps someone else experiencing this issue to diagnose the ubiquitous 5.7.1 error.