Microsoft Exchange Online Mailboxes Incorrectly Quarantined Due to Infrastructure Change

Ongoing Mailbox Quarantines

Microsoft is working to resolve an issue affecting Exchange Online users, where mailboxes have been mistakenly quarantined since Sunday, July 19. This incident, tracked as EX1436407, has resulted in users being unable to send or receive emails and access their calendars.

Root Cause Identified

The problem has been linked to a recent infrastructure change. This change led to excessive memory consumption due to unexpected indexing data, causing an out-of-memory condition that triggered the incorrect mailbox quarantines. Microsoft stated that affected users sending emails to these mailboxes may receive Non-delivery Reports (NDR).

Remediation Efforts Underway

Microsoft is actively cleaning up the excess indexing data. As of Wednesday evening, the cleanup process was 72% complete, up from 66% earlier in the day. Mailboxes are being gradually removed from quarantine as memory levels are validated across different regions. A full completion timeline has not yet been provided by Microsoft.

Previous Incidents

This is not the first time Microsoft has faced issues with email quarantines or incorrect spam tagging. In March 2025, an Exchange Online bug mistakenly quarantined some users' emails, and in May 2025, a machine learning model incorrectly flagged Gmail emails as spam. More recently, in September, an anti-spam service issue blocked URL access and quarantined emails for Exchange Online and Microsoft Teams users. Another issue in February involved faulty heuristic detection rules incorrectly flagging legitimate URLs as phishing links, leading to quarantined emails.