receiving an ndr on employee removed from exchange

Hi- we are receiving the following: The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.

this user was removed from our system. when a user in the office sends an invite to the all staff we will receive the above NDR. we are on win2k and exchange 2k and will be moving to 2003 and moving from a domain structure of a root with 2 child to a flat domain -collapsing into the root. anyone have any ideas?