Mail problems

I've just set up a Mandrake 7.0 box and all appears to work great except
e-mail.
I've installed "postfix" and the POP3 one I can't remember its name
off-hand.

I can send outgoing e-mail fine, such as root@pamarq.com

When I reply to it, I get a message back saying that the server doesn't
allow relaying.
I'm not trying to relay, just to send a message back.
How does one turn off this feature, but still only allow relaying to a
select few IP addresses?
Thanks!

Mark Myers
http://www.pamarq.com