Exchange Online Quarantining Mailboxes in Error Since Sunday

Exchange Online is quarantining customer mailboxes in error. Microsoft is investigating. The issue began approximately July 20, per BleepingComputer reporting published July 23, 2026. Resolution: pending.

Confidence

• Confirmed: Exchange Online automated quarantine is flagging legitimate mailboxes incorrectly.
• Confirmed: Incident active since approximately July 20, 2026.
• Unconfirmed: Root cause — Microsoft has not published the triggering condition.
• Unconfirmed: Scale — no affected tenant count disclosed.

What to check

If you have user reports of inaccessible mailboxes or missing email since Sunday, check the Microsoft 365 Service Health dashboard at admin.microsoft.com → Health → Service health. Affected tenants will have an open incident record there with Microsoft’s in-progress updates.

Admins can inspect and release quarantined items under Microsoft 365 Defender → Review → Quarantine, but Microsoft has not confirmed the fix is complete. Bulk-releasing quarantined mailboxes before remediation is done risks the same logic re-triggering on the same items. Unless there is a business-critical need, hold until Microsoft signals resolution and track the service health dashboard for updates.

What this isn’t

There is no CVE here. This is Microsoft’s own quarantine logic misfiring — a logic or configuration error on Microsoft’s infrastructure, not an external attacker or an exploit. The symptoms overlap with what a legitimate phishing filter might produce, so before escalating internally as a security incident, confirm the cause against the service health dashboard first.

Remediation updates will come through the Microsoft 365 Service Health dashboard before public statements. Watch the BleepingComputer report for resolution timestamps.