email account does not exist

We are using Exchange 2003 and Outlook 2003. Some users receive the following message when sending an appointment to a different person (usersa).

The following recepient(s) could not be reached:
Usreb and Date
The e-mail account does not exist at the organization this message was set to.

Userb was removed from the Network months ago. Why do we get it and how do I eliminate it?