I think the only real value in this extension would be to reward clients
that recognize it, by providing their users with more predictable
service (=less delay, more uniform delay).
When I wrote my CEAS paper on greylisting, I found that very little
legit mail is affected by it. A reasonable greylister only delays the
first message seen from an IP, and after that whitelists it. (I
realize there are greylisters that delay every new [IP,from] or
[IP,from,to] but the solution is not to do that.) Very little legit
mail comes from an IP you've never seen before.
Every discussion list I know comes from a single IP or a very small
set of IPs, so the first message or more likely the subscription
confirmation might be delayed, but nothing after that.
I really don't see anything to fix.
R's,
John