DKIM selector explained
DKIM selector · what is DKIM selector · selector._domainkey
What a DKIM selector is, where it appears in DNS, and why mail providers use different selectors for signing keys.
By DN01 Network Team
A DKIM selector is the label that tells receivers which public key to fetch before verifying a signed email message. DKIM proves that a message was signed by the sender and that the public key needed for verification is published in DNS.
Most DKIM debugging mistakes start by checking _domainkey.example.com instead of the exact selector._domainkey.example.com name published by the sender. Always test the exact selector._domainkey.domain name. The domain alone is not enough because each selector can publish a different public key.
Open /en/dkim-validator, enter the selector and domain, then compare the TXT answer with /en/dns-checker when delegation or TTL is unclear. Review the TXT answer, DKIM tags, key length, testing mode and revoked state before changing mail provider settings.
What to verify first
Confirm that the selector is copied from the mail provider, not guessed from another domain or an old migration note.
Check whether the answer is a direct TXT record or a CNAME chain that eventually points at the DKIM TXT value.
Look at v=DKIM1, k=, p=, h=, s= and t= together. A record can exist but still be unusable when the public key is empty, malformed or too weak.
Common failure causes
The record was added to the wrong DNS provider, while the domain still delegates to another authoritative nameserver.
The selector changed during a provider migration, key rotation or domain re-verification, but an older selector is still being tested.
The TXT value was split or quoted incorrectly by a DNS editor, so receivers cannot reconstruct the base64 public key.
How to fix safely
Publish the new key first, wait for DNS visibility, then switch signing in the provider console. Do not delete the old selector until delayed mail has cleared.
Use t=y only during rollout. Once signed mail verifies consistently, remove testing mode so DKIM sends a stronger production signal.
Keep DKIM checks next to SPF and DMARC checks. A valid DKIM key is one part of deliverability, not the whole authentication policy.
| Tag | Why it matters |
|---|---|
| v=DKIM1 | Identifies the TXT value as a DKIM record. |
| p= | Carries the public key; an empty value revokes the key. |
| k= | Declares the key type, commonly rsa or ed25519. |
| h= | Limits allowed hash algorithms such as sha256. |
| t= | Flags testing mode (y) or strict subdomain behavior (s). |
Frequently asked questions
- Can I check DKIM without a selector?
No. DKIM records are selector-specific, so you need the selector from your provider or from a DKIM-Signature header.
- Why is the DKIM record found but invalid?
The TXT record may have a wrong version tag, duplicate tags, missing p=, invalid base64, a revoked empty key or a weak/malformed public key.
- Should I keep the old selector during rotation?
Yes, keep it until TTLs and delayed mail queues clear. Removing it too early can break verification for messages already in transit.