Microsoft Exchange Online is Wrongly Locking Away Customer Mailboxes

Key points

• Microsoft has been fighting an Exchange Online fault, tracked as EX1436407, that has wrongly quarantined customer mailboxes since 19 July.
• Affected users can't reliably send or receive email, and senders writing to them get bounce messages back.
• Microsoft blames a recent infrastructure change that consumed excessive memory and triggered the false quarantines.
• Cleanup of the excess indexing data reached 72% by Wednesday evening, up from 66% earlier that afternoon.
• The company hasn't said which regions or how many customers are hit, and has given no completion date.

Microsoft is still working to fix an Exchange Online fault that has been locking customer mailboxes by mistake since Sunday. Exchange Online is the cloud email service businesses rent from Microsoft to run staff inboxes and calendars.

The incident, tracked internally as EX1436407, started on 19 July and wasn't resolved as of the most recent update Wednesday evening. While it runs, some users can't receive email at all, others can't send, and calendars have been out of reach. People emailing an affected mailbox get a Non-Delivery Report back, the automated bounce message confirming their message never arrived.

What actually went wrong?

A recent change to Microsoft's own back-end infrastructure consumed far more memory than expected. When servers ran out, the system wrongly flagged customer mailboxes as suspicious and quarantined them, walling them off from normal email flow.

"A recent infrastructure change caused excessive memory consumption from unexpected indexing data results in an out-of-memory condition, causing affected mailboxes to be incorrectly quarantined," Microsoft said in its advisory.

Indexing data is the behind-the-scenes catalogue Exchange builds so it can search mail quickly. That catalogue ballooned and starved the servers. Microsoft says this is a recurrence of an earlier problem it logged as EX1434354, and that the previous fix wasn't sufficient.

Who is affected?

Microsoft hasn't said. No list of affected regions has been published, and the company hasn't disclosed how many tenants or mailboxes are caught up in the fault. It has labelled the event an "incident," a classification it reserves for problems with noticeable user impact.

First reported by BleepingComputer, the outage is the latest in a string of Exchange Online mishaps where legitimate mail got wrongly held back.

How is Microsoft fixing it?

Engineers are clearing out the excess indexing data. Progress moved from 66% complete Wednesday afternoon to 72% by Wednesday evening. As memory pressure eases in each region, Microsoft is pulling mailboxes back out of quarantine.

The company hasn't committed to a completion time, saying only that a timeline would come in a later update. The next scheduled update was set for 6 p.m. UTC.

What should ordinary users do?

If your work email's been patchy since Sunday, this is likely why, and there's nothing you personally need to fix. Resend any important bounced messages once services stabilise. IT teams should watch Microsoft 365 admin centre notifications under incident ID EX1436407 for regional updates rather than opening new support tickets.

A pattern of quarantine bugs

This isn't Microsoft's first Exchange Online quarantine problem. An anti-spam bug grabbed legitimate messages in March 2025; a machine-learning filter started marking real Gmail messages as spam in May 2025; a September fault blocked Exchange and Teams users from opening links; and in February, over-eager anti-phishing rules flagged thousands of clean URLs as credential theft.

The timing makes the broader Microsoft 365 picture harder to read right now. We reported on 22 July that Exchange 2016 and 2019 go end-of-patch in October 2026, meaning customers still on older on-premises kit who are weighing a cloud move now have fresh reasons to question how cloud reliability holds up under Microsoft's own infrastructure changes.

Each of these incidents was resolved. Each also cost customers hours of missing mail while Microsoft untangled what its own systems had done.