Facebook says it will use a new standard called Require-Recipient-Valid-Since (RRVS) that gives senders the means to indicate to receivers a point in time when the ownership of the target mailbox was known to the sender. If the account changed hands since Facebooks last confirmation, Yahoo can just drop the message, preventing delivery of sensitive messages to the wrong hands. Facebook wants to help other online properties follow suit and has documented the extension via the Internet Engineering Task Force.”]