Microsoft working to fix Exchange Online mailbox quarantine issue

Microsoft is addressing an ongoing issue in Exchange Online that has mistakenly quarantined user mailboxes since July 19, 2026, due to a recent infrastructure change causing excessive memory consumption. The out-of-memory condition triggered incorrect mailbox quarantines, blocking users from sending and receiving emails and accessing calendars. This incident, identified as EX1436407, is a recurrence of a previous issue (EX1434354), and Microsoft is performing cleanup operations to restore affected mailboxes.

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