An Exchange Server 2007 or 2010 user mailbox that has been deleted may not appear in the disconnected mailbox list for the server on which it was previously held.
This can happen when the mailbox has not been deleted using the Disable or Remove commands in the Exchange Management Console. For example, if a mailbox was deleted using the Exchange 2003 management tools then it will not appear in disconnected mailboxes.
When mailboxes are deleted incorrectly the Exchange 2007/2010 mailbox server doesn’t process them correctly, and as a result they do not end up in the disconnected mailbox list.
However you can fix this problem with the Clean-MailboxDatabase command. Simply launch the Exchange Management Shell and run the command against the database that the user’s mailbox was on.
Clean-MailboxDatabase "EX2010MB\First Storage Group\Mailbox Database"
Refresh the disconnected mailboxes view for that mailbox server. The mailbox should now appear correctly and can be reconnected if necessary.