IPv6 makes publishing IP address reputations impractical. Since IP address
reputation has been a primary method for identifying abusive sources with
IPv4, imposing ineffective and flaky > replacement strategies has an effect
of deterring IPv6 use.
In practice, the /64 prefix of the IPv6 address has very much the same
"administrative" properties as the /32 value of the IPv4 address. It should be
fairly straightforward to update a reputation system to manage the /64 prefixes
of IPv6. This seems somewhat more practical than trying to change the behavior
of mail agent if their connectivity happens to use IPv6.
-- Christian Huitema