[Exim] Denying mail "From: *@domain.com" from certain ip add…

Hey,

I've been given a task of setting up our mail server so that no one outside
our netblocks can send mail to someone in our company from a company email
address, e.g.

(from a host outside our netblocks)
From: bob@???
To: jeff@???

would be denied, but if they were from a host within our netblocks (hence in
the company) it would be allowed to be sent.

Having no idea even if this is possible or not, could anyone point me in the
right direction for the features and options I should be looking at?

Thanks,
Andy.