DomainKeys
DomainKeys (informally DK) is a deprecated e-mail authentication system designed by Yahoo 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), which is now widely used. Both DomainKeys and DKIM were published in May 2007, DomainKeys as an "historical" protocol, and DKIM as its standards-track replacement.