Out of all of the things we do to restrict spam, the only one with a steady trickle of false positives is the host lookup not passing reverse DNS check.
The only place I've seen which publicly talks about the reverse DNS requirement is AOL. A huge majority of senders are correctly configured in DNS, as attested by our rate of requiring a new whitelisted IP roughly once per week, and knowing that up to 800,000 messages per day per server fail the test without upset to users. Once in awhile, someone will make a little stink about it and claim they have never run into the requirement before "in seven years". It would be nice to have a list of organizations which do require reverse DNS check or some similar fact to back up the sensibility of it all. Any thoughts or comments out there on this? --Donald
