The outgoing mail server will have to see what addresses account@yahoo.com
did send to that haven't been verified ("authenticated") yet. If it gets a
request from an account that was not sent to, then it sends an 'account does
not exist' message. This will cause the spoofed message to be rejected,
while not allowing the existence of an account to be determined by a
malicious entity.
This assumes a pretty complex infrastructure mapping between all
possible sending servers and receiving servers for the same domain.
And until everyone has updated their mail servers, the false positive
rate is going to be huge, so you don't dare block on it.