User cannot access mailbox & sending email to this type of mailbox will receive an NDR stating “Your message wasn’t delivered because the recipient’s mailbox is quarantined”


The mailbox is a potential threat to the health of the Information store and has been quarantined. There are several types of events for which the Exchange store tags a mailbox as a potential threat:

  • If a thread doing work for that mailbox fails
  • If there are more than five threads in that mailbox that haven’t made progress for a long time


This is called Poison mailbox issue, any mailbox identified as potential threat to the mailbox database will be quarantined by exchange servers, since Office365 multi tenant environment we cannot do a manual process to fix this. However by default exchange resets Quarantined mailboxes in 24 hrs after the quarantine flag is set.

How to Find the status:

Run this command to retrieve Quarantine status of the mailbox:

Get-MailboxStatistics UserName | fl *Quarantine*

IsQuarantined : True

QuarantineDescription :

QuarantineLastCrash :

QuarantineEnd : 6/18/2015 3:57:37 PM

QuarantineFileVersion :

Reference Poison Mailbox:

Post comment for questions

By Kingson Jebaraj

Microsoft MVP, Blogger, Owner and Publisher for, Microsoft TechNet Author, Solution Architect, Former Office365 Technical Lead for Microsoft(Partner) Extensive knowledge and experience in Microsoft Exchange and Cloud Messaging Services and has got more exposure on Messaging environment deployment,migration,designing and other project management activities, I have earned real time experience in handling multi-site distributed critical large environment of messaging system. Been awarded as an MVP (Microsoft Most Valuable Professional) for Office Servers and services from Microsoft for an exceptional real world contribution made through Microsoft forums and other Microsoft communities. Currently working as “Solution Architect” on Private/Public cloud and SaaS environment for Pacific Controls, UAE, Dubai. One of the largest TIER III certified green data center campus in the middle east.