It has been pointed out several times now that unless you are willing to
receive mail only from a small, closed group of individuals that all
agree to use digital signatures and whose mail you whitelist while
blacklisting EVERYTHING ELSE you are right back where you are right now.
If we can communicate the fact that a message is discarded because it was categorized as spam back to the sender without adverse side effects, then occasional false positives aren't much of a problem.
As for signing list messages: some practical experience would be good here. I'm not sure this list is the best choice to gain it, though.