[Exim] Authenticating Mail Senders

Hi

I know that Exim support SMTP Authentication using PAM.

What would be really great is if there is a possibility to match the
authenticated user against the senders mail address for outgoing mail, so
it can be ensured that no one can send a mail on behalf of somebody else.

My idea is to do this via LDAP:

dn: uid=timtas,dc=dplanet,dc=ch
objectClass: person
objectClass: inetOrgPerson
uid: timtas
cn: Tim Tassonis
givenName: Tim
sn: Tassonis
mail: timtas@???
...


So, if a sender is authenticated and he specifies his "MAIL FROM:" header,
exim could check the address simply with an LDAP Filter like this:

(&(mail=$sender)(uid=$authuser))


Is something like this already implemented or easily integratable?

If using an external transport, the only thing the transport would have to
know is the userid of the authenticated user. Is this supported somehow?

Bye
Tim