Using postfix as mail relay ?

["Sebastien Cabrol" <cabrolse@xxxxxxxxxxxxx>]

Hi,

I would like to use postfix to send and receive mail for a valid domain
(domain.com) but relay the incoming/outgoing mail to a subdomain
(domain.internal).

My subdomain (domain.internal) is sitting on a SBS 2003. I'd like to
have this server to have a minimum access to the outside world. So I was
thinking to set-up postfix to handle mail servicing.
Where postfix would forward all incoming mail to the users via
".forward" [user@xxxxxxxxxxxxxxx].
But my problem at present is "how to set-up the SMTP or mail relay
servicing?" 
Also Postfix is install on a solaris 8 server, in a private DMZ with a
real and fake IP.
Can somebody indicate me what should I configure? Should I setup a mail
relayhost anything like that?

Thanks!

Sebastien