So, in the authentication phase, Exim would try to bind to the LDAP server
by:
- Looking up the dn based on the sumitted userid, timtas
- Bind with the dn and the submitted password.
When the client sends the MAIL FROM: header, Exim would already know the
authenticated userid and would then lookup the entry using the filter
(&(uid=$AUTHENTICATED_USERID)(mail=$ADDRESS_IN_MAIL_FROM_HEADER))