Email authentication

What is DKIM?

How legitimate email carries a signature that receiving systems can verify.

A verifiable signature

A sending system adds a cryptographic signature to a message. The receiving system retrieves the corresponding public key from DNS and verifies that signature.

Selectors matter

DKIM records use selectors chosen by each sending service. There is no universal selector a public checker can reliably guess.

That is why a careful public assessment should not claim DKIM is missing simply because one guessed selector does not resolve.