[Exim] "transparent" smtp server

Lo all,

I'm currently contemplating on attempting to setup a "transparent" exim
server... I have a client with mainly laptops on their office network, and
they have very good reason to want to use a internal smtp server (Mainly due
to control / reporting / etc). Now, because these laptops are mobile, the
majority of them go home at night, and the majority of them uses dialup in
the evenings and days to send emails... It is mainly win based OSes, and
from what I can see, neither outlook nor oe has the options to specify
different smtp servers for different types of connections - and even if they
do, none that is easy accessible without spending allot of time to configure
it.

From what I saw and understand, a simple ipfw fwd on FreeBSD *should*
forward all outbound smtp sessions to a local server, and forwarding these
connections to any smtp server *should* work... What may become a problem,
is that I suspect the majority of clients will use smtp authentication on
their isp's mail servers... Thus, I am going to have a problem, as the
client's session that is forwarded to my "transparent" exim server will
issue a username / password that exim will obviously deny.

Thus my question... Is there a quick "hack" to tell exim to allow any and
all authentication (cleartext / md5 / etc) from a certain set of hosts?
Basically, allot 192.168.1.0/24 any authentication (regardless of the
username and password), but still authenticate the rest of the people
wanting to send email to through the server???

I think it should be possible with the right condition on the
authenticators... I've just never been to clued up with those conditions...

--
me