Relay of foreign mail

Is there a way to stop foreign mail arriving at my server from being relayed back out to the internet as though it was sent from my system? Have a Sun Netra i as our mail server and we have bulk mailings being relayed off of our server. Some one stated that I should just turn relay off, but unclear has how to do it, and if I do will my mail clients still be able to send e-mail out to the internet?