Exchange Online: Stupid email glitch sends too much into quarantine

Many Exchange Online users are struggling with unexpected problems. A bug suddenly moves harmless emails to quarantine and blocks access to calendars. We summarize the status of Microsoft’s troubleshooting.

Mailboxes unexpectedly in isolation

Exchange Online has had operational restrictions since July 19, 2026. A technical error also incorrectly moves inconspicuous messages into quarantine. Affected users may not be able to send or receive emails. Senders often receive automatic bounce reports. There are also outages when accessing personal and shared calendars. Microsoft is working to resolve the incident, which is reported internally as EX1436407. Companies worldwide that use the group’s infrastructure are affected.

Suspected as the cause Günter Born a change to the server architecture. This created incorrect indexing data and led to high memory consumption. As a result, an out-of-memory condition occurred, which triggered the isolation of mailboxes. Administrators should monitor affected accounts.

Bug fix progress

The issue is reportedly related to a previous incident. Microsoft is currently removing excess data from servers. By midweek, the fix was more than 70 percent complete. Affected accounts will be gradually reactivated. Status reports are available in the Microsoft 365 admin center. According to current information, there is no evidence of a cyber attack or data loss. Similar incidents occurred in February 2026 and March 2025, when emails were quarantined due to faulty filters.