DomainKeys Identified Mail (DKIM) is an Internet standard that uses public-key cryptography and DNS to provide for validation of email messages. DKIM uses a domain to digitally sign important elements of the message to ensure the message remains unaltered in transit.

Using DKIM, the receiving email system can check that an email that claimed to have come from a specific domain was indeed authorized by the owner of that domain. DKIM achieves this by affixing a digital signature, linked to a domain name, to each outgoing email message. The recipient system can verify this by looking up the sender's public key published in the DNS. A valid signature also guarantees that some parts of the email have not been modified since the signature was affixed. Usually, DKIM signatures are not visible to end users, and they are affixed or verified by the infrastructure rather than the message's authors and recipients.

TechMail, powered by Exchange Online (part of Microsoft 365), makes use of DKIM.

DMARC, which is also used by TechMail, uses DKIM as part of its checks regarding message validity.