In Exchange 2010, if an AD user is disabled but another user has access to their mailbox, will they still be able to access the mailbox or does that user need to remain enabled for this functionality to work?
Accessing a Disabled AD User’s Mailbox with Full Access
exchange-2010
Best Answer
If you disable a user in AD, their mailbox still exists and you should still be able to access the mailbox because AD simply controls the authentication, but the permissions to the mailbox will still exist. At least, that was how it was in 2003. I haven't had the opportunity to work with 2010, but I assume the functionality would be similar in this regard and easily testable.
You can also connect the mailbox to someone else's account. I'm going to link you to this technet article that can give you some ideas on what will be easiest for you; technet article!
A side note to this is this applies to disabled AD accounts as well.