First go to www.mxtoolbox.com and check to see if you domain is blacklisted. Put your domain in the lookup box and click lookup. then click the BlackList link to see if you are listed. Then get yourself
Second use your firewall to limit outbound emails to your mailservers only. If you don't know how to do this, post your firewall information here (make, model and code version if possible) and we'll see what we can do.
Third, turn on verbose logging on your receive connectors and then check to see where most of the connections are comming from.
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