postfix relay error

[william_holt at speakeasy.net (Bill Holt)]

I can send and receive mail from within the domain xample.org/(private addressing behind firewall) [this is where the mail server is...] to otherxample.net/(public address), however, when i use a client outside of the domain, I can only send within xample.org, for example, when i try to send from within xample.org to otherxample.net, i get the error: the mail server responded <someone at otherxample.net> relay access denied.
Within main.cf, I have trusted networks my.internal.sub.net
Also, I am getting pounded by the same addresses in my maillog, this would be spam right?
I'm a newbie at mail, using postfix/cyrus on rh9, thanks for any help.