[Exim] reverse lookups

Hello all,

I am curently running exim 3.6x. I already know, I should consider
upgradeing to 4.X. Once C-panel upgrades I'll be able to. Here is the
delema I face. I want to let only emails that have the correct
arpa/reverse lookup through and want others to bounce with a message
something like. Sory, but your reverse lookup doesn't match. please
contact your administrator to corect this. Our server has been
getting slammed by spam, lot's of them try and mask thier identity so
instead of blocking out RoadRunner and AOL (both known for ppl that
like to spam) I'd rather just refuse the email comming in. If you
don't have the corect entry or just don't wanna make it that easy :)
please can you get me started with the right query that'll get me
going in the correct direction.

Thanks,
--
Jerry M. Howell II