[HSE20260122-01-I] Incident: Partial service disruption: Inbound/Outbound email workflow

We are currently investigating reports of emails being rejected with a "531 cannot be resolved" error.
The root cause is under analysis. Other services are not affected.

In addition, there is an ongoing incident on Microsoft's side.
Email traffic, both outgoing and incoming, addressed to Microsoft tenants is being deferred with a "451 4.3.2 temporary server issue" error.
https://admin.cloud.microsoft/#/servicehealth/:/alerts/MO1221364

We have confirmed that the issue is external and related to Microsoft 365.

The errors "531 \ cannot be resolved" and "451 4.3.2 Temporary server error. Please try later ATTR2" occur because the M365 MX records (e.g., .*.mail.protection.outlook.com) are currently not resolving due to DNS problems on Microsoft's side. As a result, we cannot retrieve the IP addresses needed to deliver emails to Microsoft tenants.

To avoid message loss during this incident, we are temporarily converting 531 rejects into deferrals, so affected emails will be queued instead of bounced. This may increase the size of the mail queue, but ensures that no messages are rejected while Microsoft resolves their DNS issues.

Customers using the Continuity Service can continue sending and receiving emails while the issue on Microsoft's side is being resolved.