"This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online," Microsoft said in a service alert.
Incident timeline and Microsoft’s public notice
Microsoft first acknowledged the outage — tracked under incident EX1467029 — at 02:19 AM EDT, saying it was investigating reports of intermittent "Server busy" errors. The company described the problem as an ongoing Exchange Online outage that delays email sent to and received from external domains and advised that multiple mailboxes may be affected.
Microsoft has not provided a timeline for full remediation. In its service alert, the company said it was continuing to analyze service telemetry to understand the underlying cause and to determine the most effective mitigation path, and that it would provide further details later in the day.
Anti-spam protections identified as a likely aggravating factor
Microsoft's initial investigation identified instances where anti-spam protections "may be contributing to impact for a subset of users." The company warned that these protections can further delay affected accounts, implying that mitigation could require adjustments to spam-filtering behavior as part of any fix.
Microsoft said it was continuing to analyze behavior and review telemetry to better understand the root cause, indicating that any steps to change anti-spam handling would be informed by that work.
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See what we buildScope: regions and user counts remain unspecified
The company has not disclosed which regions are impacted or how many users are affected. Microsoft did flag the situation as an incident, a designation the company uses for problems that produce noticeable user impact, but provided no geographic breakdown or user counts in the public alert.
Recent pattern of Exchange Online and Microsoft 365 service interruptions
The current outage follows a string of service interruptions Microsoft publicly addressed this year. In April, Microsoft 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; that same April event also included Exchange Online mailbox access issues that intermittently affected Outlook mobile and macOS users for weeks.
In June, Microsoft addressed a similar service issue affecting mail flow for customers in North America, Asia-Pacific (APAC), and Europe. And earlier this week, on Thursday, Microsoft said it mitigated a massive outage that caused authentication issues, connection problems, service delays and failures, and other issues for Microsoft 365 customers.
What this means for technologists, affected enterprises, and end users
- Technologists and security teams: Expect to monitor Exchange Online telemetry and message queues closely; Microsoft's note on anti-spam protections suggests teams will need to watch spam-filter logs and delivery diagnostics if delays persist.
- Affected enterprises and procurement leaders: Organizations that rely on timely cross-domain email should review incident response plans and communications strategies for customers and partners while Microsoft continues its investigation — especially if external-domain mail flow is a business-critical function.
- End users and the general public: Users attempting to send or receive messages to or from external domains may experience intermittent "Server busy" errors and delivery delays; Microsoft’s advisory is to expect ongoing delays until the company provides remediation details.
Where the record ends — and what to watch next
Microsoft is still investigating the incident and will provide further details as analysis completes. The company has indicated anti-spam protections are a potential exacerbating factor but has not announced specific mitigations or a timetable for restoration. That leaves two immediate, concrete open questions: when Microsoft will complete its telemetry review and what operational changes will be required to resolve the delays for affected mailboxes.
This is a developing story; Microsoft has said it will provide additional information later today.
