Mail gateway as in outbound mail has to be relayed through another mail server? There's a few docs in the
wiki for that.
Mail gateway as in delivers to a separate mail server and you want to pull mail down from that server? You could just have that mail server forward mail destined for your users. Or, you could have the server
pull down mail for each user via POP or IMAP, but I'd have to be pretty desperate before doing that..