KB00030301 Allowing only some local users to send mail remotely
Under some circumstances you may wish to restrict certain users to only sending internal mail (i.e. not permit them to send mail to users outside of your domain). It is relatively simple to do this, if you take advantage of the fact that Mailtraq already has the means to refuse mail destined for a remote address.
The trick is then to make Mailtraq think the machine is not on the local network. Mailtraq determines if a machine is local or not by searching for its IP address in the LAN section of Options | Server.... Unless Mailtraq actually needs to directly contact a machine, it does not need to know if it is local or not (except for relaying). Thus, remove the machines from that list.
Then go to Options | Services... and open the properties for the SMTP service. In the relaying tab uncheck "Relay for client machines outside this LAN".