What do you mean by dismissed account ?
Is this account disabled or deleted ?
If the account is still enabled and you want to put a rule to let the sender's know that this employee is no longer working, you can use Transport Rules
http://www.msexchange.org/tutorials/Transport-Rules-Exchange-Server-2007.htmlSee the screenshots. you can change the from people check box to -> sent to people.
and then proceed accordingly.
hope this helps.