Authentication
What is DKIM Selector?
The label in a DKIM signature that tells receivers which public key to fetch from DNS to verify the message.
A DKIM selector is a string (for example s1, google, or k1) carried in the s= tag of a message's DKIM-Signature header. The receiver combines it with the signing domain from the d= tag to build the DNS query selector._domainkey.domain, where the matching public key lives in a TXT record (often delegated via CNAME). Selectors let a single domain publish multiple keys at once, which enables key rotation and lets different sending services each use their own key.
Why it matters for cold email
Each sending tool bolted onto a cold-email domain publishes its own selector, so a missing or mistyped selector record is a silent DKIM failure that breaks DMARC alignment even when SPF passes. When rotating keys or adding a new ESP, confirm the new selector's record resolves before you send.
Related terms
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