An e-mail 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 DomainKeys headers.

Yahoo! and Cisco = DomainKeys Identified Mail
Yahoo!'s DomainKeys was combined with Cisco's Identified Internet Mail system, which maintains signature consistency, to become DomainKeys Identified Mail (DKIM). DKIM is backward compatible with the DomainKeys system. See e-mail authentication and digital signature.