Question : Echange server sending spam...

Exchange 2003 server sending spam and has been blacklisted.
Started our of nowhere from a domaine name that was forgotten but still active and pointing to the server.
Some tests shows that the server isn't open relay re: mxtoolbox.com
How to test if the server is the one sending out? [email protected] is always the sender
How to test if it is one of the 27 station internaly that could have been infected in some way.
McAfee anti virus 8.0 everywhere and updated

Answer : Echange server sending spam...

you could be the victim of backscatter , see here for a description.(http://en.wikipedia.org/wiki/Backscatter#Backscatter_of_email_spam).
You may want to enable tar pitting to stop this: http://support.microsoft.com/kb/842851
Also, have a look here: http://www.amset.info/exchange/smtp-openrelay.asp that article and heaps of others on that site will assist you greatly.