DKIM, short for DomainKeys Identified Mail, is an email authentication standard that adds a cryptographic signature to a message. The sending mail server signs selected headers and parts of the body with a private key, and the recipient checks that signature against a public key published in DNS. If the signature is valid, it shows the message was likely sent by an authorized domain and was not changed in transit.
In cyber security, DKIM matters because email is a primary path for phishing, impersonation, and business email compromise. It does not prove that a sender is trustworthy, and it does not stop all fraud, but it helps detect spoofed or tampered mail. Defenders use DKIM together with SPF and DMARC to reduce forged messages and to enforce policy when mail fails authentication. Attackers may still abuse lookalike domains or compromised accounts, but DKIM raises the cost of simple header forgery and makes message validation more reliable.