DKIM

DKIM, DomainKeys Identified Mail, is a domain-level digital signature carried inside the message. The sending system hashes the body and a chosen set of headers, always including From:, signs that hash with a private key, and attaches the result as a DKIM-Signature header.

Two tags in that header carry the meaning. d= names the signing domain, which takes responsibility for the message and need not be the same as the domain in the From: header. s= names a selector, pointing at one specific published key: a signature reading d=mailer.net; s=k1 sends the receiver to k1._domainkey.mailer.net to fetch the public key. No certificate authority is involved, which is the main way DKIM differs from the certificate model behind TLS.

Selectors let a domain publish several keys at once. That is what makes rotation possible without a verification gap, and what lets a third-party platform sign on your behalf under its own selector. Every delegated selector is a private key you do not hold.

A verifying signature proves exactly two things: the signed content was not altered after signing, and the domain in d= vouched for it. It does not encrypt anything, so the message is readable at every hop. It signs for a domain, never for a person, which is why it does not deliver non-repudiation. And an attacker signing with their own d= passes cleanly. Because the signature travels with the message, DKIM survives the forwarding that breaks SPF, but a mailing list that appends a footer alters the body and invalidates the hash.