Resolve Sendmail Office 365 Relay

Our Sendmail server has been relaying mail through Office 365 for years without a hiccup, but as of today every outbound message fails with the log entry “stat=User unknown.” Nothing on the system was recently patched or reconfigured, so I need a Sendmail-and-Exchange-Online specialist to step in, trace the fault, and restore normal delivery.

Key areas that will probably require inspection include the local sendmail.mc / sendmail.cf settings, SMTP relay authentication, our Office 365 connector, TLS handshake parameters, any IP or domain blocks, DNS records and, of course, the mail logs on both sides. I can supply full SSH access to the mail host plus the Microsoft 365 admin portal and message trace results.

Deliverables (acceptance criteria):
• Identify the root cause and document it briefly.
• Apply the corrective change(s) or provide the exact configuration I should apply.
• Prove success with test messages and clean logs showing 2.0.0 delivery status.

There have been no recent updates or changes from our side, so fresh eyes on the configs and connectors should get us back online quickly. If you have solid, hands-on experience marrying Sendmail with Exchange Online, I’m ready to start right away.