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Issue Issues and confusion with mail forwarding

spiceypork

New Pleskian
Server operating system version
Ubuntu 22.04
Plesk version and microupdate number
18.0.74 Update #3
I am having an issue where sometimes email sent to a forward only email address on the server does not get correctly forwarded to the target
Additionally I am confused by what I see in the log files although I will admit I am not overly familiar with how the email forwarding flow is supposed to work

The scanario:-
Plesk is installed hosting mydomain.co.uk

Plesk Version
Plesk Obsidian Web Host Edition
Version 18.0.74 Update #3

Server Software
Ubuntu 22.04

Configuration
I have multiple email forward accounts set up. All are configured the same
Do not allow login to plex with the email
Mailbox size 0
Switch On mail forwarding is selected
Do not copy to inbox is selected

Two of the email recipient forwarding configs that created the most recent problem (there have been other undelivered test mails)
[email protected] -> [email protected]
[email protected] -> [email protected]

Two days ago I created both addresses and sent a single email with [email protected] and [email protected] as recipients
mydomain_user2 received the email correctly. Mydomain_user1 one did not and today I received a bounce back saying mailbox full

The testmail came from my gmail account
[email protected]

However trawling through the log chain things do not make sense to me

Here's an excerpt from the logfile

2025-12-29 14:37:10infopostfix/smtpd [862674]12A01416D4: client=mail-pf1-f169.google.com[209.85.210.169]
2025-12-29 14:37:10infopostfix/cleanup [862677]0C229416D4: milter-reject: END-OF-MESSAGE from mail-pf1-f181.google.com[209.85.210.181]: 4.2.2 Mailbox full; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail-pf1-f181.google.com>
2025-12-29 14:37:10infopsa-pc-remote [1141]0C229416D4: check-quota: stderr: DEFER
2025-12-29 14:37:10infopsa-pc-remote [1141]0C229416D4: check-quota: stderr: DATA REPLY:452:4.2.2 Mailbox full
2025-12-29 14:37:10infopostfix/cleanup [862677]0C229416D4: message-id=<CABjuiC87W+Lfa97fkz0YRe90DBQ_xPn-D29hgU8WGtGXU3y4Cw@mail.gmail.com>
2025-12-29 14:37:10infopsa-pc-remote [1141]0C229416D4: from=<[email protected]> to=<[email protected]>
2025-12-29 14:37:10infopostfix/smtpd [862672]0C229416D4: client=mail-pf1-f181.google.com[209.85.210.181]
2025-12-29 14:37:09infopostfix/smtpd [862674]connect from mail-pf1-f169.google.com[209.85.210.169]
2025-12-29 14:37:09infopostfix/smtpd [862672]connect from mail-pf1-f181.google.com[209.85.210.181]

What confuses me greatly is that from the logs it appears that [email protected] was passed to gmail instaed of [email protected]

The processing of the second recipient of the email is as follows:-
2025-12-29 14:37:10infopostfix-local [862685]12A01416D4: send message: id=S862685 from=<SRS0=1Zyd=7D=gmail.com=[email protected]> to=<[email protected]>
2025-12-29 14:37:10infopostfix/smtpd [862672]disconnect from mail-pf1-f181.google.com[209.85.210.181] ehlo=2 starttls=1 mail=1 rcpt=1 bdat=0/1 quit=1 commands=6/7
2025-12-29 14:37:10infopostfix-local [862685]12A01416D4: arc-sign: stderr: SKIP
2025-12-29 14:37:10infopostfix-local [862685]12A01416D4: from=<[email protected]>, to=[email protected]>, dirname=/var/qmail/mailnames
2025-12-29 14:37:10infopostfix/qmgr [4215]12A01416D4: from=[email protected]>, size=3570, nrcpt=1 (queue active)
2025-12-29 14:37:10infopsa-pc-remote [1141]12A01416D4: check-quota: stderr: SKIP
2025-12-29 14:37:10infopostfix/cleanup [862677]12A01416D4: message-id=<CABjuiC87W+Lfa97fkz0YRe90DBQ_xPn-D29hgU8WGtGXU3y4Cw@mail.gmail.com>
2025-12-29 14:37:10infopsa-pc-remote [1141]12A01416D4: from=<[email protected]> to=<[email protected]>

While this mail was delivered successfully I'm confused by the last chronological entry in the excerpt which seems to imply that the email was redirected to outlook with a sender who is completely unknown. It has munged together my email [email protected] with the domain name mydomain.co.uk to produce a fictitious [email protected] as the sender.

However outlook_user2@outlook replied to that message and it arrived back at [email protected] so perhaps the mail log format is just incorrectly configured (I have changed nothing) or I am misunderstanding the log file

To further cement that mis-config/understanding option I today sent another message to [email protected] only which was succesfully delivered with the correct reply to address
The log started with
1266A403A5: from=<[email protected]>, to=<[email protected]>, dirname=/var/qmail/mailnames

and ended with
7133141DCD: to=<[email protected]>, relay=n1smtpout.europe.secureserver.net[92.204.64.1]:25, delay=0.07, delays=0.01/0.01/0.01/0.04, dsn=2.0.0, status=sent (250 2.0.0 awpQvcwYax89x mail accepted for delivery)

Along the way was the created non-existent user. This is the same from report as the previous correctly delivered email
7133141DCD: uid=30 from=<SRS0=Abbk=7F=gmail.com=[email protected]>

This seemed to follow a completely different path being sent to a relay

It seems like I'm up against 2 issues
1) Either the logging is misconfigured or I am misunderstanding it because as far as I can see the From: user reported in the logs does not exist
2) Sometimes mail simply isn't delivered.
From the logs (which may or may not be correctly reporting events and/or may or may not be understood correctly) it appears that it was presented to the final destination with completely incorrect credentials. Could it be that two recipients both of which are forwards/aliases has confused the system.
Am I misunderstanding configuration and the mailbox full message is from my server despite being configured not to add emails to the inbox. And if that's the case why did the mail to mydomain_user2 complete with the same config. As well as the repeated mail to mydomain_user1 which also successfully completed

I'm thoroughly bamboozled but as always when a hardware engineer messes with software there a suspicion of PEBKAC

Regards
Spicey
 
Addendum
Typical
~20 minutes after posting [email protected] just replied to the original test message. He also reported ~3% usage on his gmail account and claims not to have deleted anything recently

What happened is still a complete mystery
The latest logs show that the mail was resent by gmail? arrived at the server and followed the relay path that all other successful messages followed

If someone could please explain what is happening/how i humped the config/Whose mailbox was full I would be eternally grateful
 
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