[Solved] Your message wasn’t delivered because the recipient’s mailbox is quarantined

Few days back at one of my client, Exchange server has started to show anonymous behavior and Database got dismounted. We had a hard time to restore that database and make it keep running. Everything was okay, but very next day my client made me a call and told me, he does have some emails as a NDR saying ‘Your message wasn’t delivered because the recipient’s mailbox is quarantined’. So, I asked for the reference email and this here is the detail.

Issue:

Email send to the several users inside the organization are getting NDR with ‘Your message wasn’t delivered because the recipient’s mailbox is quarantined’. Also the users are not able to get access to their email too.

Cause:

This is called Poison Mailbox issue, any mailbox identified as potential threat to the mailbox database will be quarantined by Exchange server.The mailbox is a potential threat to the health of the Information store and has been quarantined. In case of Exchange we can do few manual processes to restore the mailbox, but in case of O365 we cannot because of its Multi-tenant environment. Although the quarantine will get restore after 24 hrs.

Resolution:

To identify the issue, first make sure you had the proper mailbox, you can check it with the below cmdlet.

For the single user

[PS] C:\Windows\system32>Get-MailboxStatistics username |fl *quarantine*

To check in whole database, how many users are quarantined..

[PS] C:\Windows\system32>Get-MailboxStatistics -Database EXMDB01 |Select DisplayName, IsQuarantined

Basically, after 24 hours your email will be restored, but if you want to restore manually you can use below cmdlet. For O365, it will not work.

[PS] C:\Windows\system32>Disable-MailboxQuarantine test01

When you check again for that mailbox, you will find it is restored….

Reference on mailbox poisoning and store: https://technet.microsoft.com/library/bb331958.aspx