RE: Reverse DNS RFCs and Recommendations

["Tony Hain" <alh-ietf () tndh net>]

John Levine wrote:
> Right.  Spam filtering depends on heuristics.  Mail from hosts without
> matching forward/reverse DNS is overwhelmingly bot spam, so checking for
> it is a very effective heuristic.

Leading digit is clearly in widespread use beyond 3com & 1and1. One of the most effective heuristics in my acl list is:
\N^.*@\d{3,}\.(cn|com|net|org|us|asia)

In the last few hours it has picked off multiple messages from each of these:
Carol28 () 8447 com
Jeff17 () 3550 com
Ronald79 () 0785 com
Kevin57 () 2691 com
Deborah76 () 3585 com
Kimberly34 () 5864 com
Sarah94 () 0858 com
zavfdv () 131 com
qgmklyysyn () 163 com
pjpeng () 163 com
fahuyrw () 163 com
Daniel57 () 4704 com
Helen95 () 2620 com