No bounce back mail from a deleted mailbox for external mail users


we kinda got suprised with a strange issue where external customers doesn't get a bounce back (mail could not get delivered) mail from our Sophos MTA (Sophos Central E-Mail Gateway).

So they assume the e-mail got delivered successfully.

I did check if there is a remaining alias, contact or group with that specific e-mail adress which got deleted months ago but I could not find anything that belongs to the old mailbox.

If I send an email internally we also get a "bounce back reply". The problem only takes place from external incoming mails to that deleted account.
I did also check sophos central mail flow history and couldn't find anything either.

We also notice that this happened atleast twice now (more deleted mailboxes are effected but not every single one, its still rare).

How can we (as an IT administrator) make sure that this doesn't does effect even more very old deleted accounts (mailboxes).

Thanks for your help.

Edited tags
[edited by: Raphael Alganes at 3:02 PM (GMT -8) on 7 Nov 2023]