Some gory details for the interested. BLUF: If Odhiambo's message gets through, it's probably because his MTA doesn't implement the sender side of MTA-STS. mailmanlists.net does implement the receiver side, and its configuration is too obscure for me to verify that it works.
For the nerds/masochists: SMTP MTA Strict Transport Security https://datatracker.ietf.org/doc/html/rfc8461 Brie Yoder via Mailman-Users writes: > 9/8/2026 4:31:59 AM - Server at > MW4PR12MB8611.namprd12.prod.outlook.com<http://mw4pr12mb8611.namprd12.prod.outlook.com/> > returned '550 5.4.317 Message expired, cannot connect to remote server(451 > 4.4.8 MX hosts of 'mailmanlists.net<http://mailmanlists.net/>' failed > MTA-STS validation.)' (Sorry for the long lines.) That says "cannot connect ... failed MTA-STS", which implies either the host is offline or there is a network misconfiguration. The "failed MTA-STS" seems likely to be a Microsoft engineering brain bubble where the MTA-STS handler is catching an error raised by the socket manager and emitting an inaccurate message. But this > 9/8/2026 4:21:59 AM - Server at mailmanlists.net<http://mailmanlists.net/> > (0.0.0.0) returned '450 4.4.317 Cannot establish session with remote server > [Message=451 4.4.8 MX hosts of 'mailmanlists.net<http://mailmanlists.net/>' > failed MTA-STS validation.] > [LastAttemptedServerName=mailmanlists.net<http://mailmanlists.net/>] > [BL6PEPF0001E442.namprd02.prod.outlook.com<http://bl6pepf0001e442.namprd02.prod.outlook.com/> > 2026-09-08T04:21:59.309Z 08DF0B0055FEBFFE](451 4.4.8 MX hosts of > 'mailmanlists.net<http://mailmanlists.net/>' failed MTA-STS validation.)' says "cannot establish session ... failed MTA-STS." That does suggest an issue with host configuration at mailmanlists.net. They do have MTA-STS configured in their DNS, which might be causing the problem. It's complicated with a lot of aliasing and eventual delegation to tutanota.com (!), and there's no visible alias in the DNS connecting the policy-specified MX to mailmanlists.net. Steve -- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan ------------------------------------------------------ Mailman-Users mailing list -- [email protected] To unsubscribe send an email to [email protected] https://mail.python.org/mailman3/lists/mailman-users.python.org/ Mailman FAQ: http://wiki.list.org/x/AgA3 Security Policy: http://wiki.list.org/x/QIA9 Searchable Archives: https://www.mail-archive.com/[email protected]/ https://mail.python.org/archives/list/[email protected]/ Member address: [email protected]
