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."