[Exim] Authentication

Can anyone point me at the man page, document or webpage that
explains how I can use saslauthd or pam to authenticate smtp
sessions? I seem to have set hosts_auth_allow_relay = * but
I need to set up the "authenticators at the bottom of the file".

A simple PLAIN login would do, but the examples don't seem to
get their data from pam or saslauthd - instead we're looking
at /etc/exim/passwd, or coding the password (or crypt()'d
secret) right into the configuration file..

So.. please does anyone know where I should be looking for
enlightenment on this subject? Thanks.

--
Matt Sealey <matt@???>