[Exim] Relaying only for AUTHenticated users and localhost

Hello,

I'm using Exim 4.20.
I've been trying this for a while but I have not been able to configure
Exim to this:
Allow relaying of emails only for my users and only when they've used
AUTH. If the user
didn't authentify then send an appropiate message like Relaying denied
or Authentication
required (or something along those lines).
I know there are some things to take into account like accepting
incoming emails even if
no AUTH is given, allow to send emails from the localhost without auth
(php and the like),
and maybe some other situation I'm might not already be aware of.

Is there a simple way to make exim behave in this way?
Would this behavior be RFC compliant?

I don't want to allow sending emails by any other means... You must
either
be a process running in the server or use SMTP AUTH in your email
client.

Any ideas of how this is done? I've already read the documentation a few
times but
I just don't seem to get it right. I've managed to get this behavior but
the
message I get from the server when not using AUTH is not the one I've
specified and
sometimes contains garbled characters (meaning I did something wrong).

Thanks!

- Eleazar Martínez