[Exim] Forwarding SMTP AUTH (RFC 2554)

Hi.

The task at hand is to use Exim 4 as SMTP relay for internal users.

The problem is: the users are required to authenticate themselves
by using SMTP AUTH (RFC 2554) before sending mail out. But the
whole user database is stored under another mail server, which
currently can authenticate them thru SMTP AUTH.

Thus, I hope Exim's support for SMTP AUTH provides a way to
forward the authentication request to another server. I mean,
in order to evaluate an SMTP AUTH request, Exim would
repeat the same request to another server; if that server
accepts the authentication, then Exim should also
permit the relay.

I'm checking the Exim documentation for such a feature, but
I'm not quite familiar with the authentication mechanism
of Exim.

Maybe someone could point if such "SMTP AUTH forwarding"
is at all possible with Exim, and if so, give some
pointers on the proper part of the documentation
(or suggest a small sample config, which I could use
as start point).

Many thanks,
Everton