[Exim] Exim for root mail only

Hi,

I have 2 machines I am remotely administering for which I want exim to
only send root mail. I do not want it to accept any mail from other
hosts, which is always only spam and ends up getting frozen for the
lack of a local part.

The root mail is to go to my ordinary user account on the machines, from
where it is handled by a .forward to my account on my home machine.

I am looking for the config to stop exim accepting external mail but so
far cannot find it.

Can anyone give a quick answer, or point me to where I should be
rtfm-ing?

TIA,

--
David

Quidquid latine dictum sit, altum sonatur.