I've noticed anecdotally that many times when a legitimate source has no PTR on the mail host, or has a PTR with no matching A record, the domain does have a SPF record authorizing the host. This carelessness is probably born from ignorance or overwork. The PTR pass could be used to balance out the PTR fail. I have not had a chance yet to test this out in real mail flow to see how close it comes to being something good enough to reject mail.
Joseph Brennan
