[Exim] SMTP Auth based on domain names @ non local hosts

Hi All,

I`m sure this is something someone can run off the top of their heads
but I`m trying to do the following (and due to deadlines placed upon me,
if someone can pull it off the top of theirs heads I`d be highly grateful).

I need a rule to state if a message is sent from user x@??? (where x
is the local domain) from a non-local host (ie: y.com) use SMTP auth
to verify the sender.

Ideas ?

Chris