We experienced significant delays with delivering outgoing email email to domains hosted on Microsoft 365.
Delivery is back to normal.
Original report
Email is being throttled (no blocked), suggesting a temporary problem. Throttling usually occurs if Microsoft has detected a pattern of undesirable messages from the sender (i.e. our servers). The issue is affecting delivery from some, but not all our servers.
We have reached out to Microsoft Support and are awaiting a response.
Update 23:00 UTC
We have received official confirmation that our server IP address is not blocked. However, the throttling persists, effectively working the same as a complete block.
We have partially mitigated the issue by changing the sending IP address for several high-volume domains. The full solution remains working things out with Microsoft, and right now it seems virtually impossible to get through to them. Please reach out to us at support@anno.com if you have important email that is unable to deliver to a domain hosted at Microsoft 365.
Update 04:00 UTC
Managed to reach the Office 365 Support Team!