Exchange Gurus - Email Alias

if the mailbox has been deleted then the user no longer exists in exchange and there would be no alias to use would be my guess.

If the user still exists then what you want wouldn't be an issue at all, we do it regularly for employees that have left/been terminated, but once the mailbox is deleted it's all gone.

ah I see what you're saying now. You want to basically have a dummy alias created with the old person's email address to be used with a user...I'm not sure the answer to that.

Maybe has something to do with legal? We have to keep people in the system (emails etc) until we're sure there is no lawsuit ongoing or pending before we can delete anything, so maybe they need everything left alone. Hell I don't really know.

You're trying to add the alias to the old employee? As in Mr.disabled@company.Com you want to add an additional alias to that account? or you want to add the alias for Mr disabled's old email address to yours? If the user mailbox is active then you can add aliases to them unless there is some department policy forbidding it. You can't add an alias to another account when it is already being used for another. In addition, I'm pretty sure even if the account is disabled in ad the mailbox can still be active.

Edit
Rereading I think I see you want to add Mr.disabled as an alias to your mailbox. As long as that mailbox has been deleted you should have no problem doing so. Again, there may be policy forbidding it but it's technically possible.