Reclaiming Yahoo! Usernames In A Way That’s Secure: Require-Recipient-Valid-Since | YDN Blog — Yahoo!
If a Facebook user with a Yahoo! email account submits a request to reset their password, Facebook would add the Require-Recipient-Valid-Since header to the reset email, and the new header would signal to Yahoo! to check the age of the account before delivering the mail.I don’t expect all the sites adopting this, but sounds like a good workaround for the specific problem.