Un-Authorized email being sent from Exchange 2003

Hi Everyone,

I am running a Windows 2003 Standard server (All up to date with Service Packs) and Microsoft Exchange 2003 Enterprise SP2.  We noticed the server was getting bogged down, the SMTP queue was enormous, and we keep getting blacklisted.  We have about 80 mailboxes on the server.  Everyone uses Outlook 2003 or 2007.  20 accounts are Exchange clients and the rest are popping.  I checked and double checked that we are not an open relay.  We have run numerous virus scans on the desktops and the server.  We spent 7 hours with a Microsoft tech and the response is Exchange doesnt have a way of showing who has authenticated and sent mail.  We are now using a paid smart house (AuthSMTP) to send mail (Had to do it because of the blacklisting).  In their report options I can see that the server is sending mail  FROM:  email accounts that are not any of the 5 domains hosted on the Exchange server.

Any help would be appreciated.

Thanks