mail filtering based on reverse DNS

[Tim Chown]

Our MXes have been dual-stack for a long time and reject mail from systems that have no reverse DNS entries.  We would expect MX operators to include reverse entries.

The only time I know for sure that that bit us was when the IETF tried its secondary/backup facility for a few days, and they hadn't added reverse entries for the IPv6 MXes, so we rejected IETF mail.  Unfortunately the IETF servers did not retry over IPv4, so I for one dropped off some IETF lists.

It would be interesting to know what proportion of spam can currently be 'intelligently' dropped by enforcing a reverse lookup.  How applicable does the technique remain today?

Tim