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...
 
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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
 
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