mail filtering based on reverse DNS

[SM]

Hi Tim,
At 07:19 11-08-2011, Tim Chown wrote:
>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.

If your (IPv6) mail server returns a 550, there won't be a retry over IPv4.

Regards,
-sm