Definition: DKIM

(DomainKeys Identified Mail) An email authentication method that computes a digital signature which is added to the message header. The receiving mail server obtains the sender's public key from the DNS system to validate the signature. In 2004, Yahoo! began to sign all outgoing mail with DKIM headers, and other mail services followed.

Yahoo! and Cisco = DKIM
Yahoo!'s DomainKeys was combined with Cisco's Identified Internet Mail system, which maintains signature consistency, to become DKIM. DKIM is backward compatible with the DomainKeys system. See email authentication, Vouch By Reference and digital signature.