[exim] Relayed Mail: sender domain in localdomains?

Hi all,

I'm still trying to wrap my head around Exim syntax, but what I'm trying
to do is:

a) accept mail destined for local domains from other MTAs as usual.
b) relaying: authenticated users: require their reply-to/from match an
entry in /etc/localdomains, and
c) only let them use a domain that is associated with their username
(not other user's domains)

I'm not even sure if (c) is possible, but I would really appreciate
ideas on accomplishing (a) and (b).
I will continue to dig around the documentation of course. Exim is just
new territory for me :)