[Exim] SMTP Authentication Using LDAP

Hi Everyone.

Has anyone been able to use LDAP to Authenticate uses via SMTP.
I have a Large Userbase (250 000+) users within an LDAP Database with which
I would like to Authenticate before sending mail. I have setup Exim to
become a relay server for our International Roaming
clients, having this solution in place requires some sort of control to
prevent unauthorized spamming.

I found some examples which use PAM/NIS/CRAM to do the authentication for 2
different types of Email clients.
Does anyone have an idea how I could adapt it to query a LDAP server rather
than a NIS / PAM etc server.

If anyone could lend a hand It would be greatly appreciated.