DKIM Selector
The label that identifies which DKIM public key to use, published at selector._domainkey.example.com.
A DKIM selector is a short label (like `google`, `mail`, or `s1`) that points to a specific public key in DNS. When a mail server signs a message with DKIM, it adds a header naming the selector and the signing domain, for example `d=example.com; s=google`. The receiver looks up `google._domainkey.example.com` to fetch the public key and verify the signature. Selectors let a domain rotate keys without breaking older signed mail and let multiple providers (Google, Mailchimp, SES) each publish their own key under different selector names without conflict.