Have you ever wondered why there are so many disconnected mailboxes? That has flagged a big security alert in our firm, especially after finding out how IT staff used to disable user accounts! By disabling a mailbox, you are actually detaching that mailbox form it’s AD object, this orphaned mailbox is prone to deletion according to your Exchange mailbox retention policy (by default 30 days!).
We have also found few other mailboxes for active users but they are sitting in Disconnected Mailbox, by running the command:
Get-MailboxStatistics -Server ServerName | where {$_.DisconnectReason -eq ‘SoftDeleted’}
The result would show user display name for those mailboxes that had moved from one DB to another. Exchange would mark the source mailbox as SoftDeleted rather than the default Disabled – a mailbox gets flagged as Disabled when disabling mailboxes using the Disable command within the MS Exchange GUI or Disable-Mailbox via Powershell.
SysCloud.dev 
#1 by Dave M on 3 December, 2013 - 11:01 am
Most useful Sam, I used this just today.
Thanks and keep up the good work.