Starting from sometime yesterday (cannot be sure when without digging through logs) mail is now bouncing from my server.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
 
For further assistance, please send mail to <postmaster>
 
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
 
                  The mail system
 
<BOUNCE@<domain>> (expanded from <admin@<domain>>): unknown user: "bounce"
 
Final-Recipient: rfc822; BOUNCE@<domain>
Original-Recipient: rfc822;admin@<domain>
Action: failed
Status: 5.1.1
Diagnostic-Code: X-Postfix; unknown user: "bounce"

I haven't changed / installed / updated anything between when it last worked and today. bites nails

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Solution:

I've seen that on some cPanel migrations, where the user "bounce" is setup as a blackhole... except, their way of handling all that doesn't come over cleanly and generates these weird messages 🙂

It looks like the user "admin@domain" is configured to forward messages to "bounce@domain" -- and bounce@domain is generating, well, bounce messages.

A better way to handle that would be to simply remove the unwanted aliases altogether, and allow Postfix to handle the rejection messages during the SMTP greeting.

My suggestions would be:

• Go into "Edit Mail Aliases", and make sure everything in there goes where you want it to

• Remove any reference to "bounce@", as well as any aliases pointing to it. The bounce will be handled in the SMTP greeting

• Make sure you aren't using a catch-all address; catch-alls generate tons of bounce messages 🙂