Mail Relay in Exchange 2000

I can’t seem to understand how to disable relaying on my sever while allowing people to send emails properly. I have users that travel around the globe, so I don’t know what IP’s they’ll be coming from. All my users are using POP so I guess the option of allowing relaing to those who authenticate is not the right one either.
And when I check to allow only “the list below” I have to specify all the domains I permit sending mail to. It’s kind of imposible considering that there ar eover 100,000,000 registered domain names.
I know there must be a way around this (that’s why the option is there) but how do I use it?
Thanks for any advice,
Victor