[exim] LDAP-based relay control

I have a box running Exim-4.66, ClamAv, DSPAM and SpamAssassin placed
in front of a mail server running Scalix. I'm trying to configure Exim
to do look up whether a user mailbox exists on Scalix via LDAP.
I currently don't have access to the box, so I can't post my
ldap_relay router, but I'd like ideas from anyone who's implemented
this before.

Regards,
Steve