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Microsoft addresses Exchange Online mailbox quarantine incident

Feeds.4Sysops •IT News • July 23, 2026

Microsoft is currently resolving a service incident, tracked as EX1436407, which has caused Exchange Online mailboxes to be incorrectly quarantined since July 19. The issue stems from a recent infrastructure change that triggered excessive memory consumption during indexing, leading to an out-of-memory state. This disruption prevents users from sending or receiving emails and accessing calendars, often resulting in non-delivery reports for external senders. Source

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This article is one source in a clustered incident — the cluster page carries the summary, timeline and every other outlet covering it.

Microsoft Resolves Exchange Online Mailbox Quarantine Issue Microsoft is addressing an Exchange Online incident, tracked as EX1436407, which began on July 19, 2026. The issue has caused mailboxes to be incorrectly quarantined due to excessive memory consumption from a recent infrastructure change. Affected users are unable to send or receive emails and access their calendars, resulting in non-delivery reports for external senders. As of July 23, the cleanup process is ongoing, with 72% of the excess indexing data removed. Microsoft has not disclosed the number of affected customers or the specific regions impacted. This incident is a recurrence of a previous issue, EX1434354, indicating a pattern of similar problems. Microsoft plans to provide a completion timeline in a future update. 2 sources 53

Microsoft addresses Exchange Online mailbox quarantine incident

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