Microsoft works to resolve Exchange Online outage causing email delays

Microsoft confirmed an ongoing outage in its Exchange Online service that was causing intermittent "Server busy" errors and delaying email sent to or received from external domains. The incident, tracked as EX1467029, was first acknowledged at 02:19 AM EDT when the company began investigating user reports.

"This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. Users may experience intermittent 'Server busy' errors, affecting multiple mailboxes," Microsoft said in a service alert.

The company advised that affected clients should automatically retry sending messages, and that senders who received a non-delivery report after 24-72 hours would need to resend the email.

"Our initial investigation has identified instances where anti-spam protections may be contributing to impact for a subset of users. We are continuing to analyse the behaviour and review service telemetry to better understand the underlying cause and determine the most effective mitigation path," the statement added.

Microsoft did not disclose the geographic scope or the number of users affected, but classified the event as an incident, indicating noticeable impact. Earlier outages in April, June and earlier this week had similarly affected mail flow and authentication for Microsoft 365 customers.

An update at 10:26 EDT on September 4 reported that the issue had been resolved for new emails, with telemetry showing normal service performance and organizations confirming the problem was no longer occurring. Microsoft said it was monitoring email queues and conducting final validations to ensure full resolution.