Craig Morrison Mail Transport System Professional (aka MTS Pro) acts as an open relay when configured to relay all mail through an external SMTP server, which allows remote attackers to relay mail by connecting to the MTS Pro server, then sending a MAIL FROM that specifies a domain that is local to the server.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24985
http://www.vupen.com/english/advisories/2006/0786
http://www.securityfocus.com/bid/16840
http://www.securityfocus.com/archive/1/426181/100/0/threaded