Question : What's the easiest way to track down the source of internal spam?

My organization is being blacklisted by a few places and it is exceedingly difficult to determine why.  Here are the particulars:

Exchange 2003 SP2
Sonicwall 1260 Pro firewall
~50 PCs using Outlook to connect to Exchange
Symantec Corporate AV 10

Here are the things that I have tried already:
Blocked all outgoing SMTP traffic from everywhere but the Exchange server on the firewall.
Confirmed current Virus Defs on all PCs and ran a full scan.
Ensured that open relay was disabled on the server.

Anyone have any ideas on what I might be doing wrong here?  Is there a simple way to find out who is generating unwanted messages?

Answer : What's the easiest way to track down the source of internal spam?

I would take those bets on it being SPF, because that isn't the cause :)

The problem is the mailserver hostname - you can change that link this -> http://www.block.net.au/help/smtp-greeting

You want it to be mail.realogicin.com and not mail.rla.realogicinc.com

That should do it.

CBL will blacklist you for this, which is probably why you had a problem