sendmail and a firewall

[David Richards]

hey
       we have this firewall, well not sure if u can call it a firewall, but
it is stop me from doing some stuff with my freebsd box.
what the problems is, that i can't send mail to anyone? only local users on
the machine. When i try sending an email to user else where, i just get a
error send not such user, but i know there is a email, but i am writing from
the address now. I am trying different methods of getting it to work?? but
one solutions i am thinking about is nat? will i need to use nat to route
the mail to a port on my relay host ? or something ?
any ideas ? free feel to tell me

regards 
david