I sure understand the problem that the scarcity-based address reputation hack won’t work that well on IPv6. But the “solution”… And NIST isn’t exactly Mom and Pop’s Bait and Internet Shop.
Nor is gmail, which also requires that incoming IPv6 mail be authenticated with SPF or DKIM. They know what they are doing, and they have decided that the amount of legit mail they will lose by doing this is insignificant compared to the improvement in the amount of spam and malware they will be able to filter.
You can complain about how stupid they are or you can upgrade your mail system to use authentication methods that have been IETF standards for a decade and get your mail delivered. Your choice.
Regards, John Levine, johnl@xxxxxxxxx, Taughannock Networks, Trumansburg NY Please consider the environment before reading this e-mail. https://jl.ly PS:
No idea whether there is code around that can react to a DEFER with a change of source address.
Unless it changes to an IPv4 address, it won't help.