Domain_Keys
DomainKeys (informally DK) is a deprecated e-mail authentication system designed by Yahoo[1][2] to verify the domain name of an e-mail sender and the message integrity.
Aspects of DomainKeys, along with parts of Identified Internet Mail, were combined to create DomainKeys Identified Mail (DKIM),[2][3][4] which is now widely used.[5]
Both DomainKeys and DKIM were published in May 2007, DomainKeys as a "historical" protocol, and DKIM as its standards-track replacement.