[exim] LDAP lookup and cocanation

Hi,

I'm trying to figure out a way how I can do the following:

Email is delivered to joe.bloggs@domain (the form firstname.surname@domain)

I want Exim to receive this mail, do an LDAP lookup for the record that
matches firstname=joe AND surname=bloggs and return the field
userid=jbloggs, so that I can deliver mail to the maildir
/maildirs/{userid}/.

I've got it delivering mail to maildirs, and creating the maildirs on
the fly, using a a file and lsearch. What I want to do now is extend
that to LDAP and firstname.surname expansion.

Anyone have any pointers?

Thanks,

Ben