Domain Keys Identified Mail (DKIM) is an email security standard that helps detect whether messages are altered in transit between sending and receiving mail servers.

DKIM uses public-key cryptography to sign email with a responsible party's private key as it leaves a sending server. The receiving server uses a public key published to the DKIM's domain to verify the source of the message, and that the parts of the message included in the DKIM signature have not changed since the message was signed.

Once the signature is verified with the public key by the receiving server, the message passes DKIM and is considered authentic.