Google is now using an e-mail authentication technology called DomainKeys to keep phishers from luring Gmail users to fake eBay and PayPal Web pages in order to steal usernames and passwords.
The DomainKeys technology uses cryptography to verify the domain of the sender of an e-mail. It allows e-mail providers to validate the domain from which an e-mail originates, and it enables easier detection of phishing attempts by helping identify abusive domains.
Yahoo already use DomainKeys to protect Yahoo Mail users with eBay and PayPal accounts from phishing attempts. The DomainKeys technology is covered by a patent assigned to Yahoo. [source]