User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online.

More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes.

Depending on the client, the service should attempt to periodically retry sending the email previously impacted by the issue. If the initial sender received an NDR after 24-72 hours (different clients or email providers can vary), they will need to send a new email.

Current status: Telemetry shows that the service is performing as expected for new emails and this is in line with organizations confirming that the issue no longer occurring. We’re monitoring email queues and performing final validations to confirm the issue is fully resolved.

Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues.

Start time: Friday, September 4, 2026, at 9:00 AM UTC

Preliminary root cause: The enforcement of a specific anti-spam model is contributing to impact for a subset of email traffic.

Next update by: Friday, September 4, 2026, at 3:00 PM UTC