I recently started using an LDAP addressbook, and given my correspondents are automatically added into it, I decided to use it for whitelisting in my Exim user filter.

Firstly I added an entry into the LDAP server for exim and assigned it read access to the Addressbook database.

Then I added the following into my user exim filter file. I've placed it below the entries dealing with mailing lists, so lookups only happen for non-list (or "direct") email.

if "${lookup ldap {\
      user=\"cn=exim,dc=example,dc=com\" pass=TRUSTNO1 \
      ldap://localhost/o=Addressbook?cn?sub?(mail=${address:$h_From:}) \
      }{yes}{no}}" is "yes"
then
   #logwrite "userfilter_whitelist_ldap ${address:$h_From:}"
   save /var/mail/foo/bar/whitelist/
   finish
endif

Basically the filter extracts an email address from the "From:" header (alternatively the $reply_address or $sender_address variables could be used) and checks to see if there is a corresponding name listed in the database.