Microsoft is working to resolve an ongoing Exchange Online outage that is delaying email sent to and received from external domains.

The company first acknowledged this incident (tracked under EX1467029) at 02:19 AM EDT, when it began investigating reports of intermittent "Server busy" errors.

"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 seen by BleepingComputer.

"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."

Microsoft says these ongoing email issues may be aggravated by anti-spam protections, which further delay affected accounts.

The company did not provide a timeline for full remediation, saying it's still investigating the issue to understand the root cause and will provide further details later today.

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

While Microsoft has yet to disclose which regions are impacted by this ongoing outage or how many users are affected, it has flagged it as an incident, which usually involves noticeable user impact.

In April, Microsoft also resolved an Exchange Online outage that prevented users from accessing mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols. and Exchange Online mailbox access issues that intermittently affected Outlook mobile and macOS users for weeks.

More recently, in June, it addressed a similar service issue affecting the mail flow for customers in North America, Asia-Pacific (APAC), and Europe.

Earlier this week, on Thursday, it also mitigated a massive outage that caused authentication issues, connection problems, service delays and failures, and other issues for Microsoft 365 customers.

Update September 04, 10:26 EDT: Microsoft says that the EX1467029 issue has been resolved for new emails.

"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," Microsoft said. "We're monitoring email queues and performing final validations to confirm the issue is fully resolved."