"Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour," the company said in an admin center service alert.
Microsoft incident EX1331830: initial detection and classification
Microsoft acknowledged a widespread service issue affecting the Exchange Online mail flow pipeline at 10:33 a.m. EDT, logging the event under the internal tracking number EX1331830. The company began investigating after receiving a stream of customer reports on social media and has classified the situation as an incident — a designation it says typically applies to critical service issues with noticeable user impact.
Error messages observed in the field
Customers and diagnostic telemetry have shown at least two distinct failure modes. Some users reported temporary SMTP deferral errors that include the message: "The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel." Other users encountered abrupt connection terminations with the message: "Connection was closed abruptly (SuspiciousRemoteServerError)}}". Microsoft engineers are reviewing these reports to isolate which error messages are associated with which failure patterns as they work toward a root-cause determination.
Geographic impact: North America and Germany
The company said the ongoing issues affect Exchange Online customers across North America and Germany. Microsoft noted it is reviewing additional reports from customers in those regions "to further our understanding of the current impact scenario, to isolate error messages affected users are receiving, and to determine our next troubleshooting steps." The practical effect reported by Microsoft: attempts to send or access email are resulting in extensive delays or outright failures for affected customers.
Operational consequences and current state
Microsoft's advisory describes delays measured in hours for some messages; specifically, the company reported some email messages have remained undelivered for over an hour. Engineers are actively investigating the stream of reports and working to discover the root cause. At the time of Microsoft’s advisory, the company had not announced a remediation timeline but had begun triage activities centered on error-message isolation and impact assessment.
Context: related Exchange Online and Microsoft service incidents
Microsoft's notice comes after a sequence of recent service disruptions the company reported resolving. In April, Microsoft said it had addressed Exchange Online mailbox access issues that intermittently affected Outlook mobile and macOS users for weeks, as well as an Exchange Online outage that prevented access to mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols. More recently, on Monday, Microsoft addressed an outage that prevented Teams and Office for the web users from opening files, and a separate incident that prevented customers from setting up multi-factor authentication (MFA) or accessing the My Sign-Ins platform.
How IT administrators, enterprises, and end users are positioned
- IT administrators and operations teams: Microsoft’s public advisory and the EX1331830 tracking code are the focal points for troubleshooting and status monitoring. Admins are receiving the specific SMTP and connection-closure messages cited by Microsoft and are watching the admin center service alert for progress on the company's isolation and remediation steps.
- Affected enterprises and IT buyers: Organizations relying on Exchange Online saw mail flow delays and failures in North America and Germany, and will be tracking Microsoft’s engineering updates to determine when normal mail flow resumes and whether any customer-side mitigation steps are recommended.
- End users: Individuals who send or receive mail through Exchange Online may experience significant delays or temporary failures; Microsoft’s advisory notes some messages remained undelivered for over an hour during the incident.
Microsoft engineers are actively reviewing reports to discover the root cause and determine next steps. Until the company publishes a remedial update to EX1331830 in the admin center, impacted customers should follow Microsoft’s alerts for changes to mail flow status and for any recommended operational actions.
Original report: https://www.bleepingcomputer.com/news/microsoft/microsoft-exchange-online-outage-causes-email-delays-failures/
