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DKIM selector

published · September 13, 2026

A DKIM selector is the label that names one of a domain's signing keys; the receiver looks up <selector>._domainkey.<domain> in DNS to find the public key that verifies a signature.

A DKIM selector is the label a signing server writes into the signature header to say which of the domain's keys it used; the receiver joins it to the domain as <selector>._domainkey.<domain> and reads the public key from that DNS name. Selectors let a domain run several keys at once — one per sending service, or an old and a new one during a rotation — which is why Google signs under google, Microsoft 365 under selector1 and selector2, and marketing platforms under names of their own.

What Spaw answers

Because a selector is only revealed inside a signed message, a DNS-only check can test known names but never enumerate them. Spaw asks for a fixed list of the common selectors and reports the ones that publish a key in dkim_selectors. A domain that signs under an unusual name shows an empty list, and the pages say so rather than calling it unsigned.

What it does not mean

Finding a key under a selector says a service is set up to sign for the domain; it does not say the domain still uses that service, or that its messages align under DMARC.

Related

markdown version: /glossary/dkim-selector.md

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