Facebook and Yahoo have developed a mechanism to prevent the owners of recycled email addresses from hijacking accounts that were registered on other sites using those addresses in the past.
The practice of recycling email addresses has been criticized by security and privacy experts because it opens up the door to abuse. Attackers could register deleted addresses and take over accounts on third-party sites that use them for confirming password change requests. In addition, the recycled addresses might continue to receive messages containing sensitive information that is destined for their previous owners.
Facebook's security team studied the impact of email address recycling for the site's users and has worked with Yahoo to mitigate the potential security risks. Employees from the two companies have developed a mechanism that involves adding a new field in the header of sensitive email messages to include the date since the sender has known the recipient's address.
The email provider can check if the receiving account has changed owners since the date specified in this field, and if it has, it can block the message from being delivered because it was likely intended for a previous owner.
The new field is called Require-Recipient-Valid-Since and is defined as part of an extension to the Simple Mail Transfer Protocol (SMTP) called RRVS. For now the mechanism is used by Facebook and Yahoo, but the new SMTP extension was published as a proposed standard by the Internet Engineering Task Force and can be adopted by others as well.