[exim] Mysql or ldap lookup

Hi all

I would like to build a system where by the MTA, will relay all mail, will do a lookup at against either a SQL, or more preferably a LDAP server. Basically this machien will sit in front of my
exchange machine, but I dont want it to just relay all mail, I would like it at SMTP time to check for the existance of the user / email address.

If anyone has any tips or even better a working example, it would be most helpful.

Kind Regards
Brent Clark