Sometimes an Exchange mailbox will get quarantined due to mailbox threads hanging. Reasons may be corruption, AV scanning, backups, storage issues and all manner of things.
For Exchange 2013 onwards:
Check which mailboxes are quarantined:
Get-Mailbox | Get-MailboxStatistics | Where {$_.IsQuarantined -eq $True} | fl DisplayName
Scan for corruption:
New-MailboxRepairRequest -mailbox <logon_id_of_mailbox> -corruptiontype Aggregatecounts,searchfolder,provisionedfolder,folderview
Check Repair Progress:
Get-MailboxRepairRequest -mailbox <logon_id_of_mailbox>
Re-enable mailbox:
Disable-MailboxQuarantine <logon_id_of_mailbox>
