The Require-Recipient-Valid-Since Header Field
Facebook, Inc.
This document defines an email header field, Require-Recipient-Valid- Since, to provide a method for senders to indicate to receivers the time when the sender last confirmed the ownership of the target mailbox. This can be used to detect changes of mailbox ownership, and thus prevent mail from being delivered to the wrong party. The intended use of this header field is on automatically generated messages that might contain sensitive information.