Facebook and Yahoo have teamed up in an effort to devise a secure method for handling expired email accounts, yielding a standard that could have broad benefits for other websites, as well.

The somewhat lugubriously named Require-Recipient-Valid-Since (RRVS) standard grew out of concerns stemming from a decision Yahoo made last year to begin making expired email addresses and IDs available for new registrations.

At the time, the company described the move as an effort to free up long-dormant logins so others could use them. Many were concerned that the move would allow attackers to claim old Yahoo logins and use them to hijack Facebook and other Web accounts originally associated with the email addresses. The fear was that without additional protections, attackers could use a recycled email address to request a password reset from any online account that might have been previously associated with the same email account.

“If a Facebook account were connected to a recycled Yahoo email address, that account could be taken over by the new Yahoo account owner via a password change request,” noted Facebook software engineer Murray Kucherawy in a blog post announcing the new email standard.

Kucherawy added that they wanted to closely study the shift to ensure they understood the impact it could have on Facebook.

The new Facebook and Yahoo RRVS email standard marks the culmination of that effort. RRVS is basically an extension of the Simple Mail Transfer Protocol (SMTP). It provides a way for Facebook to insert a time stamp within a password recovery email indicating when it last confirmed ownership of a Yahoo account.

If a new Yahoo account owner attempts to reset a Facebook password, the time stamp in the recovery email will let Yahoo know when, exactly, the email address was last used to do the same thing. If the previous reset request happened before the account changed hands, Yahoo will drop the message to prevent the new account owner from changing the password and taking control of the Facebook account.

The RRVS header field allows social networking sites to request a new type of validation from Yahoo when sending an automated email message to a specific user, according to Bill Mills, a member of Yahoo’s Developer Network.

Facebook’s approach is to insert a time stamp in the RRVS header to validate the age of a Yahoo account. However, others can use their own rules to determine the age of an email recipient’s account, Mills noted.

To make the approach available to other Web properties, Facebook has publicly documented the new SMTP extension via the Internet Engineering Task Force (IETF). The method has been adopted as an IETF Proposed Standard and is currently being reviewed for broad adoption.

The new standard is designed for use with any automatically generated message, including those related to account statements and password change requests, the IETF said. It provides a generic way for senders to verify whether an email address associated with a specific account might have changed hands since the address was last verified.

The validation capability is an important step forward for Internet security because it is not all that unusual for email addresses to be recycled. For instance, an email account might be reassigned to a different person in a work setting or through re-registrations of expired emails, the IETF said. Often, such changes take place without any notice.

Banks, e-commerce sites and others that emailed the previous owner of an account may not know the account had been reassigned. Therefore, they could end up sending messages containing sensitive information to the wrong recipient.

The method specified by RRVS offers a way around this issue.

“In effect, the sender is saying, ‘I know that the intended recipient was using this address at this point in time. I don’t want this message delivered to anyone else,’ ” the IETF said.