[EXIM] Using LDAP for Authorization

Hello all....
I am going to be setting up a university E-Mail system that has 10,000+ users.
They want to allow relaying only after a user has been authenticated by the
system. I am going to use LDAP for this. I also want to use exim, as well, does
anyone have any recommendations for a LDAP server, and or any hints in setting up
exim in the manner?
Thanks for the help,
Paul Yahnig