Error : said: 451 4.4.0 detected undelivered mail to <address>

D

Dep1911

New Member
Feb 13, 2026
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After migrating to Mail Gateway 9.0.6, we received the following errors: Error: 451 4.4.0 detected undelivered mail to <address> from one of the addresses.
The system consists of two cluster nodes and forwards email from a specific domain to the Zimbra mail server.
Here's the tracking center output.
Code: 
2026-02-13T00:39:11.601032+03:00 proxmoxmail postfix/smtpd[3677890]: connect from unknown[10.1.1.31]
2026-02-13T00:39:11.686836+03:00 proxmoxmail postfix/smtpd[3677890]: Anonymous TLS connection established from unknown[10.1.1.31]: TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256
2026-02-13T00:39:11.749931+03:00 proxmoxmail postfix/smtpd[3677890]: B6FD443A5C: client=unknown[10.1.1.31]
2026-02-13T00:39:11.751163+03:00 proxmoxmail postfix/cleanup[3681053]: B6FD443A5C: message-id=<698e487d.c20a0220.18d451.c107@mx.google.com>
2026-02-13T00:39:11.755583+03:00 proxmoxmail postfix/qmgr[800016]: B6FD443A5C: from=<sender>, size=6763, nrcpt=1 (queue active)
2026-02-13T00:39:11.810608+03:00 proxmoxmail postfix/smtpd[3677890]: disconnect from unknown[10.1.1.31] ehlo=2 starttls=1 mail=1 rcpt=1 bdat=1 quit=1 commands=7
2026-02-13T00:39:11.839443+03:00 proxmoxmail postfix/lmtp[3681055]: B6FD443A5C: to=<recipient>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.07, delays=0.01/0/0.05/0.02, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to <recipient> (in reply to end of DATA command))
2026-02-13T00:44:36.864324+03:00 proxmoxmail postfix/qmgr[800016]: B6FD443A5C: from=<sender>, size=6763, nrcpt=1 (queue active)
2026-02-13T00:44:37.020301+03:00 proxmoxmail postfix/lmtp[3681447]: B6FD443A5C: to=<recipient>, relay=127.0.0.1[127.0.0.1]:10024, delay=325, delays=325/0.09/0.05/0.01, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to <recipient> (in reply to end of DATA command))
2026-02-13T01:11:23.874584+03:00 proxmoxmail postfix/qmgr[3734503]: B6FD443A5C: from=<sender>, size=6763, nrcpt=1 (queue active)
2026-02-13T01:11:24.094978+03:00 proxmoxmail postfix/lmtp[3734511]: B6FD443A5C: to=<recipient>, relay=none, delay=1932, delays=1932/0.21/0/0, dsn=4.4.1, status=deferred (connect to 127.0.0.1[127.0.0.1]:10024: Connection refused)
2026-02-13T01:46:23.106502+03:00 proxmoxmail postfix/qmgr[3734503]: B6FD443A5C: from=<sender>, size=6763, nrcpt=1 (queue active)
I checked all the settings, but I can't find the reason for the undelivered emails and the accumulation of these emails in the queue.
 
Last edited:
please share the complete journal of those mails - the messages from pmg-smtp-filter are missing for example.
 
The journal is working, and I can see information about incoming emails, but journalctl -u pmg-smtp-filter | grep B6FD443A5C
doesn't output anything.
 
After running postqueue -f
I see the following messages in the logs:
Code: 
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789923]: 2026/02/13-12:59:33 CONNECT TCP Peer: "[127.0.0.1]:38850" Local: "[127.0.0.1]:10024"
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789925]: 2026/02/13-12:59:33 CONNECT TCP Peer: "[127.0.0.1]:38862" Local: "[127.0.0.1]:10024"
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3731646]: Starting "1" children
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789923]: 231CA698EF605173CA: message has ambiguous content
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789923]: fast exit because of errors (free 277385216 bytes)
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789925]: 23410698EF6051A163: message has ambiguous content
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789925]: fast exit because of errors (free 277385216 bytes)
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3731646]: Starting "1" children
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789929]: 2026/02/13-12:59:33 CONNECT TCP Peer: "[127.0.0.1]:38864" Local: "[127.0.0.1]:10024"
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789929]: 2341D698EF6059C418: message has ambiguous content
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789929]: fast exit because of errors (free 277385216 bytes)
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3731646]: Starting "1" children
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789931]: 2026/02/13-12:59:33 CONNECT TCP Peer: "[127.0.0.1]:38876" Local: "[127.0.0.1]:10024"
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789931]: 2345A698EF605BD3F5: message has ambiguous content
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3789931]: fast exit because of errors (free 277385216 bytes)
Feb 13 12:59:33 proxmoxmail2 pmg-smtp-filter[3731646]: Starting "1" children
 
Everything's great, I found a solution:
A

[SOLVED] Thread 'detected undelivered mail to'

I'm having trouble with two domains that send messages to my users.

The messages are stuck in the postfix queue with the following message:

D76C12740B 178921 Tue Feb 3 19:22:31 msprvs1=20494iL6NYVzL=bounces-321255@domain.com
(host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to <user@domain.com.br> (in reply to end of DATA command))
user@domain.com.br

DC07D26A55 84919 Tue Feb 3 11:12:29 0100019c23d8f34b-c9c6d85a-eb76-4aa4-abfa-2780936b3aa0-000000@ses.domain.com.br
(host 127.0.0.1[127.0.0.1] said...
 
  • Like
Reactions: mrgoodweather, laurensb, algorges and 1 other person
You've listed the first thing that gets checked. I found a solution to the problem, which I wrote about in the message above and provided a link to the post with the solution.
Code: 
Configuration / Mail Proxy / Options - Accept Broken MIME Structure -> Yes
 
Dep1911 said:
Everything's great, I found a solution:
A

[SOLVED] Thread 'detected undelivered mail to'

I'm having trouble with two domains that send messages to my users.

The messages are stuck in the postfix queue with the following message:

D76C12740B 178921 Tue Feb 3 19:22:31 msprvs1=20494iL6NYVzL=bounces-321255@domain.com
(host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to <user@domain.com.br> (in reply to end of DATA command))
user@domain.com.br

DC07D26A55 84919 Tue Feb 3 11:12:29 0100019c23d8f34b-c9c6d85a-eb76-4aa4-abfa-2780936b3aa0-000000@ses.domain.com.br
(host 127.0.0.1[127.0.0.1] said...
Click to expand...
Thanks for sharing your solution. I’ve run into the same issue on PMG 8.2.10 and tried to find more information about this option in the documentation, but there doesn’t seem to be anything published yet. Does anyone know when this will be added to the documentation? It would also be useful to know from which version this option became configurable and what the previous default setting was.
 
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