[exim] Whitelist policy based on reply

Hi all,

A client of mine requires "sender verification" ie someone mails
x@??? and it sends a message to simply reply then they get added to
the whitelist (I advised against it but this is what they want)
anyhow, its extremely easy to do this with procmail as their are loads
of scripts available which do just this, however the problem is he
wants this machine to be infront of his current mail server and pass
on to his existing mail server therefore no local delivery will be
done ie bypassing procmail, so im wondering how easy is it to
integrate something like this into exim.

I do not want to try and reinvent the wheel if there are sample configurations
which I can modify to suite my needs... any help greatly appreciated!

David Jacobson