DomainKeys
taken from http://en.wikipedia.org/wiki/DomainKeys
DomainKeys is an e-mail authentication system designed to verify the DNS domain of an e-mail sender and the message integrity. The DomainKeys specification has adopted aspects of Identified Internet Mail to create an enhanced protocol called DomainKeys Identified Mail (DKIM). This merged specification is the basis for an IETF Working Group which guided the specification toward becoming an IETF standard.
The DKIM standard was issued in May 2007. The DomainKeys draft was also issued under “historical” status at the same time.