[exim] Using Active Directory to verify recipient addresses

Hello

I've been using Exim for a while to verify recipient addresses via AD.

Recently I've discovered I don't need to hard code the GC address, and
instead I can use

${lookup dnsdb{srv=_gc._tcp.domain.org}{$value}fail}

Now this isn't working particularly well. It seems I need to extract the
hostname and port (3268) from the DNS SRV reply.

Can anyone point me in the right direction as to how I'd achieve this?
Google has found two pages where

others have tried this in the past, but they don't seem to work for me.

http://www.mail-archive.com/exim-users@exim.org/msg20960.html

http://lists.exim.org/lurker/message/20061004.114648.76f354a4.en.html

Cheers

Mike