This has the same problem as all of the other duct tape authorization schemes -- it breaks a lot of valid e-mail, ...
In this particular case, the biggest issue is forwarders, ...
This gets into the discussion of what percentage of mail a user gets that is like this. It varies widely.
Agreed. Around here (Ithaca NY) it's probably on the order of 20% due to all of the Cornell grads who still use their cornell.edu address.
Pick a solution or solutions that you like, or not. Virtually all of them will result in some sort of reduction in the current ability of anybody being able to send mail as anyone from anywhere.
Right. That's why for widespread deployment we have to look at the small minority of schemes that don't break legitimate mail. That's why I'm looking at CSV, which makes it easier to assign reputation to sending hosts, and domainkeys (or whatever it's called when they're done mixing in IIM) which if sensibly used makes it easier to whitelist mail from people you like. R's, John