[exim] rejecting un-resolvable IPs at smtp time

In an effort to reduce spam, I would like to defer (rather than outright refuse) reception
on any message that comes from a server without a PTR.

I am not sure how to create an ACL to check for a non-existent PTR/unresolvable IP.
I haven't found in FAQs this question.

Advice would be appreciated.
Thanks,