The signer adds a DKIM-Signature header naming its domain (d=), a selector (s=), the signed headers (h=), a hash of the canonicalized body (bh=) and the signature (b=). The receiver fetches the key from the TXT record at selector._domainkey.domain, recomputes the body hash and verifies the signature over the canonicalized headers. Algorithms are rsa-sha256 and ed25519-sha256 (RFC 8463); RFC 8301 forbids rsa-sha1 and requires keys of at least 1,024 bits, recommending 2,048.
DKIM authenticates the signing domain, not the visible From address; DMARC ties the two together through alignment. Signatures break when intermediaries modify signed content (mailing-list footers, subject tags, some security gateways), which is why ARC exists, and a valid signed message can be re-sent to new recipients (DKIM replay).
The IETF DKIM working group, rechartered in February 2025, is designing a successor (DKIM2) that signs every hop and binds the envelope; it was still an Internet-Draft in 2026. The DKIM debugger verifies a message step by step in the browser.