by Ken Task.
If you inspect the headers of what messages one gets via a Moodle forum think you'll see that sendmail and postfix (which is the default mailer for version 6.4, I think) is used as an MTA (mail transfer agent) ... ie, mail flows from localhost to where ever/how ever pointed. Sendmail involved in EMail notifications coming from the system itself to the root user and postfix for messages destined for other systems.
Also inspect the servers maillogs (/var/log/maillog) for how email traffic is routed/sent.
'spirit of sharing', Ken