In fact there's even a suggested protocol for it. I don't recall the draft
name, but it works by putting the address list in the DNS. You then use zone
transfer to move the data around and keep it up to date.
Remember the reference? I was thinking of something like that, but if
it's already done, all the better.
Sorry, I don't recall it and The Google seems to be singularly unhelpful
In any case, I'm also very interested in this and would be happy to work
on it with you if you like.
There are also at least two, and probably more, "lookahead" milters that use
part of an SMTP transaction to perform early address validation. (I use one
these myself as part of providing backup MX service for a couple of people.)
I understand about callforwards, but it seems to me that the backup MX
is mostly going to get mail when the main MX isn't available, so I
don't see how this would work very well in practice.
Actually, it works pretty well in practice. The main problem with secondary
MXes is that spammers send messages to them even when the primary is up (which
in most cases is 99+% of the time). So even if this only provides effective
filtering when the primary is up, it's sufficient to prevent most blowback.
P.S. One of the secondaries I provide is for a system that's connected via a
fairly long radio link. The link goes wonky when someone uses a cordless phone
near either end. But according to my stats, even that server manages 99.8%