Providing a fallback would negate the security benefit, obviously -- but might gather enough data for a whitelist (ish) of servers/sending domains which we can demand valid certs for.

I'm not too concerned about bouncing valid email, as long as it doesn't create loops. Nor am I that worried about missing emails.

As long as all the senders are a) mailing-lists that either should do the right thing, or will actually change to do the right thing if an error is reported, b) big free services like gmail/yahoo/outlook that might be divided into groups of "does the right thing" and "blacklist"/bounce to let any friend know that they need to send from "free service y, not x" -- and c) friends that run their own email servers and can be beaten (eh, educated) to compliance.

I'd really not like to rely on some random CA list, though. I'm personally a cacert.org-fan/user -- but I suppose one might try to lean on DANE to avoid that particular hairball -- to some extent. It's either that or manual whitelist+trust on first use/some kind of pinning.