A Brief History of DKIM

Created in the year 2004, DKIM is a culmination of 2 previously existing standards: 

Enhanced DomainKey (created by Yahoo!)

This email verification system created by Yahoo! attested to the integrity of an email message by verifying the DNS domain from which the message originated.

Identified Internet Mail (created by Cisco)

This email authentication standard introduced by Cisco used digitally affixed signatures in the message body to verify the legitimacy of outgoing messages. 

Yahoo, Gmail, AOL, and FastMail were the first in line to implement the protocol, however, in recent times it is a widely adopted and strongly recommended standard for all email providers and users.