In Exchange, when you delete an active directory user account, it does not delete their mailbox automatically. Instead it considers the mailbox to be in a “disconnected” state. The mailbox exists but it is no longer associated to an active directory user account. There are several reasons why you might want to keep the mailbox around and perhaps eventually reconnect it. Today I was working on a very corrupt user account in AD, but the mailbox itself was fine. I simply deleted the user account from AD (after ensuring proper backups were taken), and then recreated a new user account. Now even though the username is the same as the one I just deleted, they contain a different GUID, so they are, in fact, different users. After creating the AD user account, I went over to the Exchange Management Console and the users mailbox was missing from both the Mailbox list, as well as the Disconnected list. The reason for this is because these are moved during a mailbox maintenance process. However you can speed this up.
Launch the Exchange PowerShell and run the following
After that is complete, go back to the Disconnect Mailbox list and refresh the page, and you will find your mailbox.