Scenario:

I was trying to export multiple mailboxes to PST files in bulk using a CSV file and ended up with error on 3 mailboxes. The error stated, the mailbox quarantined and unable to open message store.

Cause:

One of the reasons could be that Exchange identifies if a particular mailbox is consuming a lot of CPU power at a particular time, maybe because it has some bad items or unindexed items or corruption or may have a huge mailbox size. Upon identifying such mailboxes, Exchange server will put these mailboxes in Quarantine to avoid further consumption.

When a mailbox is quarantined, it becomes inaccessible to the user and senders will receive an NDR stating the user is quarantined.

Solution:

Quarantine mailboxes cannot be removed from ECP just like Exchange Online.

You need to remove the keys from Registry Editor. Follow below steps to remove the mailbox from quarantine.

• Fetch the GUID of the mailbox.

• Now fetch the GUID of databases where the mailboxes are hosted.

• Now open regedit.exe and navigate to "HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<ServerName>\Private-{db guid}\QuarantinedMailboxes\{mailbox guid}"

• Under QuarantinedMailboxes you will find GUID of mailboxes that have been quarantined. You can delete those keys and exit regedit.

• Now you need to restart IIS for it the take effect. To reset IIS, type IISRESET /Restart

This can fix your issue. Alternatively, instead of restarting IIS you can dismount and remount the database or apart from this you can also modify the crash count value in the CrashCount Key to 2.

A second method is to run the following command in EMS:

Disable-MailboxQuarantine <Mailbox Name>

Now you can perform repairs on the mailbox using New-MailboxRepairRequest cmd.