OT:Sendmail Help

[Marco Obaid]

Few months ago, I moved my main mailserver mail.mydomain behind a mail gateway,
gw.mydomain. All mail sent from mail.mydomain goes through my gw.mydomain and
ofcourse gw.mydomain is my MX for my domain, which forwards clean feed to my
users located on mail.mydomain.

Everything is running great except one annoying thing, spammers are still able
to contact mail.mydomain.

How can I configure my mailserver main.mydomain (running sendmail) to refuse
smtp connections except when to my mail gateway (gw.mydomain)?
I do not want my mail server to talk to any server  but my gateway.

It is probably a very simple thing, I just cannot find it in my books.

Thank you for any hints
Marco