To check all server:
Get-Mailbox | Get-MailboxStatistics | Where {$_.IsQuarantined –eq $True}
Single :
Get-MailboxStatistics "User_Name" | fl IsQuarantined
Release :
Disable-MailboxQuarantine"User_Name"
Delete regkey :
HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<ServerName>\Private-{dbguid}\QuarantinedMailboxes\{mailbox guid}
Fix mailbox :
New-MailboxRepairRequest -mailbox "User_Name" -corruptiontype Aggregatecounts,searchfolder,provisionedfolder,folderview