Unable to parse message - part did not end with expected boundary

I

info.it

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Proxmox Subscriber
Mar 24, 2022
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We are encountering undelivered mail, here is the mail log. Please advise.

2026-02-24T17:33:42.645461+08:00 pmg01 postfix/smtpd[2294121]: 9D88A800D8F: client=058177177248.static.xxx.com[SourceIP]
2026-02-24T17:33:42.646691+08:00 pmg01 postfix/smtpd[2294134]: 9DD6F800D92: client=058177177248.static.xxx.com[SourceIP]
2026-02-24T17:33:42.672243+08:00 pmg01 postfix/cleanup[2296207]: 9D88A800D8F: message-id=<{508E092E-8BE7-4EEB-BA56-9042990D4B52}@HKFILE>
2026-02-24T17:33:42.674041+08:00 pmg01 postfix/cleanup[2296457]: 9DD6F800D92: message-id=<{508E092E-8BE7-4EEB-BA56-9042990D4B52}@HKFILE>
2026-02-24T17:33:42.674966+08:00 pmg01 postfix/qmgr[1305247]: 9DD6F800D92: from=<Sender-Domain>, size=9552, nrcpt=1 (queue active)
2026-02-24T17:33:42.675162+08:00 pmg01 postfix/qmgr[1305247]: 9D88A800D8F: from=<Sender-Domain>, size=9553, nrcpt=1 (queue active)
2026-02-24T17:33:42.681469+08:00 pmg01 pmg-smtp-filter[2296740]: 2026/02/24-17:33:42 CONNECT TCP Peer: "[127.0.0.1]:52574" Local: "[127.0.0.1]:10023"
2026-02-24T17:33:42.681878+08:00 pmg01 pmg-smtp-filter[2296755]: 2026/02/24-17:33:42 CONNECT TCP Peer: "[127.0.0.1]:52582" Local: "[127.0.0.1]:10023"
2026-02-24T17:33:42.688161+08:00 pmg01 postfix/smtpd[2294121]: disconnect from 058177177248.static.xxx.com[SourceIP] ehlo=2 starttls=1 mail=1 rcpt=1 data=1 quit=1 commands=7
2026-02-24T17:33:42.718517+08:00 pmg01 postfix/smtpd[2294134]: AF583800D97: client=058177177248.static.xxx.com[SourceIP]
2026-02-24T17:33:42.732056+08:00 pmg01 pmg-smtp-filter[2296740]: 800D98699D7076B0A91: unable to parse message - part did not end with expected boundary
2026-02-24T17:33:42.733062+08:00 pmg01 pmg-smtp-filter[2296740]: fast exit because of errors (free 319549440 bytes)
2026-02-24T17:33:42.734243+08:00 pmg01 pmg-smtp-filter[2296755]: 800D99699D7076B1647: unable to parse message - part did not end with expected boundary
2026-02-24T17:33:42.734405+08:00 pmg01 pmg-smtp-filter[2296755]: fast exit because of errors (free 324345856 bytes)
2026-02-24T17:33:42.737216+08:00 pmg01 postfix/lmtp[2296621]: 9DD6F800D92: to=user1@x.onmicrosoft.com, relay=127.0.0.1[127.0.0.1]:10023, delay=0.1, delays=0.04/0/0.05/0.01, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to user1@x.onmicrosoft.com (in reply to end of DATA command))
2026-02-24T17:33:42.740424+08:00 pmg01 postfix/lmtp[2296588]: 9D88A800D8F: to=user2@x.onmicrosoft.com, relay=127.0.0.1[127.0.0.1]:10023, delay=0.1, delays=0.04/0/0.05/0.01, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to user2@x.onmicrosoft.com (in reply to end of DATA command))
2026-02-24T17:33:42.742574+08:00 pmg01 postfix/smtp[2296223]: connect to x.mail.protection.outlook.com[2a01:111:f403:cc09::]:25: Network is unreachable
2026-02-24T17:33:42.745870+08:00 pmg01 postfix/cleanup[2296207]: AF583800D97: message-id=<{508E092E-8BE7-4EEB-BA56-9042990D4B52}@HKFILE>
2026-02-24T17:33:42.747585+08:00 pmg01 postfix/qmgr[1305247]: AF583800D97: from=<no-reply=28=Sender-Domain>, size=9552, nrcpt=1 (queue active)
2026-02-24T17:33:42.749478+08:00 pmg01 pmg-smtp-filter[2296730]: 2026/02/24-17:33:42 CONNECT TCP Peer: "[127.0.0.1]:52584" Local: "[127.0.0.1]:10023"
2026-02-24T17:33:42.760324+08:00 pmg01 postfix/smtpd[2294134]: disconnect from 058177177248.static.xxx.com[SourceIP] ehlo=2 starttls=1 mail=2 rcpt=2 data=2 rset=1 quit=1 commands=11
2026-02-24T17:33:42.799371+08:00 pmg01 pmg-smtp-filter[2296730]: 800D98699D7076C128A: unable to parse message - part did not end with expected boundary
2026-02-24T17:33:42.799525+08:00 pmg01 pmg-smtp-filter[2296730]: fast exit because of errors (free 324620288 bytes)
2026-02-24T17:33:42.808607+08:00 pmg01 pmg-smtp-filter[2264638]: Starting "1" children
2026-02-24T17:33:42.808929+08:00 pmg01 postfix/lmtp[2296624]: AF583800D97: to=user1@x.onmicrosoft.com, relay=127.0.0.1[127.0.0.1]:10023, delay=0.09, delays=0.04/0/0.04/0.01, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to user1@x.onmicrosoft.com (in reply to end of DATA command))
2026-02-24T17:33:43.037940+08:00 pmg01 postfix/smtp[2296223]: Trusted TLS connection established to x.mail.protection.outlook.com[52.101.157.80]:25: TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (secp384r1) server-signature RSA-PSS (2048 bits) server-digest SHA256
2026-02-24T17:33:44.894430+08:00 pmg01 postfix/smtp[2296223]: 9140F800D95: to=user3@x.onmicrosoft.com, relay=x.mail.protection.outlook.com[52.101.157.80]:25, delay=2.3, delays=0.05/0/0.49/1.8, dsn=2.6.0, status=sent (250 2.6.0 7a1f13a53fec474003e0d1dbc7cb6747@xxx.com [InternalId=70965744650124, Hostname=KL1PR0601MB5598.apcprd06.prod.outlook.com] 21480 bytes in 0.326, 64.334 KB/sec Queued mail for delivery)
2026-02-24T17:33:44.895799+08:00 pmg01 postfix/qmgr[1305247]: 9140F800D95: removed
 
Also having similar issue, started just few days ago with some specific messages: 'unable to parse message - part did not end with expected boundary'

2026-02-26T06:56:33.715462+00:00 pxmg01 postfix/pickup[258149]: AE703252F5: uid=105 from=<0101019c955bb781-d1675325-e0b2-45d7-972c-8f69bc857812-000000@us-west-2.amazonses.com> orig_id=64D04250AE
2026-02-26T06:56:33.715698+00:00 pxmg01 postfix/cleanup[260343]: AE703252F5: message-id=<0101019c955bb781-d1675325-e0b2-45d7-972c-8f69bc857812-000000@us-west-2.amazonses.com>
2026-02-26T06:56:33.720627+00:00 pxmg01 postfix/qmgr[814]: AE703252F5: from=<0101019c955bb781-d1675325-e0b2-45d7-972c-8f69bc857812-000000@us-west-2.amazonses.com>, size=37322, nrcpt=1 (queue active)
2026-02-26T06:56:33.721908+00:00 pxmg01 pmg-smtp-filter[260569]: 2026/02/26-06:56:33 CONNECT TCP Peer: "[127.0.0.1]:44256" Local: "[127.0.0.1]:10024"
2026-02-26T06:56:33.764181+00:00 pxmg01 postfix/lmtp[260246]: AE703252F5: breaking line > 998 bytes with <CR><LF>SPACE
2026-02-26T06:56:33.769185+00:00 pxmg01 pmg-smtp-filter[260569]: 250AE699FEEA1BA713: unable to parse message - part did not end with expected boundary
2026-02-26T06:56:33.771483+00:00 pxmg01 pmg-smtp-filter[260569]: fast exit because of errors (free 316432384 bytes)
2026-02-26T06:56:33.779759+00:00 pxmg01 pmg-smtp-filter[248175]: Starting "1" children
2026-02-26T06:56:33.779875+00:00 pxmg01 postfix/lmtp[260246]: AE703252F5: to=<user@somedomain.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=56645, delays=56645/0/0.04/0.01, dsn=4.4.0, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.4.0 detected undelivered mail to <user@somedomain.com> (in reply to end of DATA command))
 
See https://forum.proxmox.com/threads/detected-undelivered-mail-to.180292/post-840426 for a bit more details - else this change is due to:
ProxmoxSecurityAdvisory

Post in thread 'Proxmox Mail Gateway - Security Advisories'

Subject: PSA-2026-00005-1: Bypass of mail filters through confusion of the MIME Parser​


Advisory date: 2026-02-17

Packages: pmg-api, libmime-tools-perl

Details: The parser initially processing e-mails for further analysis was set to not cause an error on non-standard and ambiguous information in the 'Content-Type' header:
- multiple occurrences of a 'Content-Type' header
- mulitple boundary parameters for multipart messages
- backslash as part of the boundary parameter for multipart messages
- non-printable and non ASCII characters as part of the boundary...
and you can allow such mails - and then selectively decide which broken mails you want to accept based on the added header - see the reference documentation:
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmgconfig_mailproxy_broken_mime

I hope this helps!
 
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