Microsoft Exchange Online Mailboxes Mistakenly Quarantined in Ongoing Outage
Microsoft is actively working to resolve an issue affecting Exchange Online that has been mistakenly quarantining customer mailboxes since Sunday, disrupting email and calendar access for affected users.
Microsoft is currently addressing a significant outage impacting its Exchange Online service, which has led to the incorrect quarantine of customer mailboxes. The issue, identified internally as EX1436407, began on July 19th and has persisted, preventing affected users from sending or receiving emails and accessing their calendars.
The root cause has been attributed to a recent infrastructure change that inadvertently triggered excessive memory consumption. This led to an out-of-memory condition, which in turn caused the service to mistakenly quarantine mailboxes. Microsoft stated that users attempting to send emails to these quarantined mailboxes are receiving Non-delivery Reports (NDRs).
This incident is a recurrence of a previous problem, EX1434354, indicating that the initial remediation steps were insufficient for a complete resolution. Microsoft has acknowledged the need for additional steps to fully restore service and prevent future occurrences.
While Microsoft has not disclosed the specific regions affected or the total number of customers impacted, the classification of the event as an "incident" suggests a noticeable level of user disruption. The company is actively engaged in cleaning up the excess indexing data that contributed to the problem. Progress has been reported, with the cleanup moving from 66% to 72% complete, and mailboxes are being gradually removed from quarantine as memory levels are validated across different regions.
As of the latest update, Microsoft has not provided a definitive timeline for full remediation, promising further updates. The company's next scheduled update was slated for later today at 6 p.m. UTC, indicating an ongoing effort to resolve the complex issue.
This event is not an isolated incident for Microsoft's email services. In recent years, the company has faced several similar issues where legitimate emails were mistakenly quarantined or flagged as spam. For example, in March 2025, an Exchange Online bug caused anti-spam systems to misclassify user emails, and in May 2025, a machine learning model incorrectly identified emails from Gmail as spam. More recently, in September, an anti-spam service issue blocked access to URLs and quarantined emails for Exchange Online and Microsoft Teams users, and in February, faulty heuristic detection rules designed to combat phishing campaigns incorrectly flagged thousands of legitimate URLs.
The ongoing nature of these issues highlights the complexities of managing large-scale cloud email services and the challenges in fine-tuning automated security and administrative processes to avoid unintended consequences for legitimate users. The company's continued efforts to resolve this latest outage underscore the critical importance of email service availability for businesses worldwide.