Spam problem

We recently were affected by spam in our office.   Our ISP blacklisted us because of it.  We block the port 25 through out the office.  Only the exchange server can communicate with port 25.  Today I realized that the exchange queue is still loaded with spam messages.   How can that be?  Can spam use someones outlook account to send mail through the exchange?  What can I do to remedy the situation.  How can i find the culprit...
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It seems pretty evident that a virus or worm or spambot or whatever is on one of your machines.  

If I'm correct, SBS 2003 will allow any machine on the LAN to send through it without authentication.  But then again, how do we know you're not an open relay?

Step 1:
Enter your server in here:

Step 2:
Run antivirus tools on all your PCs.  Also run malwarebytes on each PC (

Alan HardistyCo-OwnerCommented:
Please have a read of my article and see if you have issues outlined in my article (Authenticated Relay predominantly):'t-send.html

LilganstaAuthor Commented:
It was not a ndr attack so I had to use an antispam service...  from that point on everything was ok.
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