The modern handbook on DKIM

DKIM adds a cryptographic signature to email. Receivers use the sender's public DNS key to verify that signed parts of the message were not changed in transit.

A selector is the DNS label receivers use to find the right DKIM public key. It lets one domain publish multiple keys for different senders or key rotation windows.

For DMARC, yes. A DKIM signature can pass technically, but DMARC only counts it when the d= signing domain aligns with the visible From domain.

Rotate DKIM keys periodically and whenever a key might have been exposed. Keep old and new selectors live during the transition so queued mail can still validate.

DKIM commonly fails because the selector is wrong, the DNS record is missing or malformed, the public key is truncated, or a gateway modified signed headers or body content.