DKIM (DomainKeys Identified Mail):

DKIM stands for DomainKeys Identified Mail and is used for the authentication of an email that’s being sent. It is an email security standard designed to make sure messages aren’t altered in transit between the sending and recipient servers.

It uses public-key cryptography to sign email with a private key as it leaves a sending server. Recipient servers then use a public key published to a domain’s DNS to verify the source of the message, and that the body of the message hasn’t changed during transit. Once the signature is verified with the public key by the recipient server, the message passes DKIM and is considered authentic.

DomainKey :

DomainKeys is a domain-level authentication standard that uses public/private key encryption, DNS, and multiple policies to prove the legitimacy and contents of an email message. It verifies the domain used in the “from” or “sender” header. An email message signed with DomainKeys will utilize both a public key record, and a policy record.

DomainKeys is a deprecated email authentication system designed by Yahoo to verify the domain name of an email sender and the message integrity. Aspects of DomainKeys(DK), along with parts of Cisco ‘s Identified Internet Mail (IIM), were combined to create DKIM (DomainKeys Identified Mail) which provides more security and flexibility.