I'm running a Microsoft Exchange 2003 (Service Pack 2) server and Microsoft
Windows XP with Outlook 2003 desktops.

With only one of my Users, when she sends out a meeting request (either from
her personal calendar or from a Public Folder calendar) end-users see the
request from the Proper name (Jane Doe). However, when the end-user replies,
the end-user gets an Undeliverable: Delivery Status Notification (Failure)
and the reason is:

The following recipient(s) could not be reached:

John Smith on 10/1/2009 12:41 PM
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. (PO1.companyname.com #5.1.1

John Smith was an employee a long time ago, but his Active Directory account
and mailbox has been purged from the system long ago. Also, if I put Jane
Doe on a new PC, the same issue occurs.

Somewhere in the Exchange/mailbox properties, every time Jane Doe send a
meeting request and someone acknowledges, the acknowledgement is going to a
dead e-mail address.

Any help would be appreciated.