Very newbie relaying question

[Declan Grady]

Hi folks,
I've just rebuilt my mailscanner+sendmail box using debian etch, and
after a lot of head-scratching got it up & running ok.
(Old box suffered hardware failure, was a redhat 7 with a lot of patches
& updates, etc, but was handy for upgrading mailscanner from rpms.)

While watching the spam messages, I've noticed quite a few supposedly
coming from my own domain name.

I'm 99% sure I can restrict it so that if the sender is supposed to be
in my own domain, it must be from a local IP address, otherwise reject
it.

I have googled, but get lost in all the authentication stuff.

My mailscanner box is a gateway - just takes incoming mail, scans &
passes on to a windows exchange box, using a sendmail mailertable.

What do I need to do to my sendmail config to permit mail from my domain
to be only accepted from internal IP's ?

I'm guessing I need to change my /etc/mail/access file somehow ?
Currently it has (among other things)

mydomain.com             RELAY
localhost.mydomain.com   RELAY
mail.mydomain.com        RELAY
mailserver.mydomain.com  RELAY
mydomain.ie              RELAY
exchange_server_name     RELAY

Obviously some of these are not necessary, and are from my tweaking it
trying to get it working. 

Or, is there some clever way to do it.


Thinking out loud, all mail from mydomain will come from the exchange
server, which has a single fixed IP address - Mabye that is a way to do
it ?

Thanks for suggestions.