Tool

DKIM check

Tests the common selectors (default, google, selector1/2, k1, mail and others) and reports where a DKIM public key was found. We automatically try the 11 most common selector names.

Permalink: ?domain=… works too.

What is DKIM?

DKIM (DomainKeys Identified Mail) complements SPF with a cryptographic signature. While SPF only checks whether the sending IP is authorized, DKIM signs selected header fields and the mail body with a private key. Receiving servers fetch the corresponding public key from DNS and verify, if the signature matches, the mail is guaranteed to be unaltered and to originate from a system with access to the private key.

The public key is stored in a TXT record at the hostname <selector>._domainkey.<domain>. The selector is a freely chosen name, common ones are default, google, selector1/selector2, k1, mail. Multiple selectors in parallel enable key rotation without downtime and independent keys per sender.

An incoming mail carries a DKIM-Signature: header line with selector (s=), signing domain (d=) and the signature. The receiver looks up the public key, verifies cryptographically and reports the result as dkim=pass or dkim=fail. For DMARC alignment, d= must additionally match the From domain.

Dive deeper → What is DKIM?

Common DKIM mistakes

  1. Selector inactive, but DNS entry still there

    Sending runs on a new selector, the old one is orphaned in DNS. Check tools report it as valid even though it no longer signs anything. Clean up, otherwise the record looks confusing when debugging.

  2. Key length only 1024 bit (RSA)

    Standard before 2015, too weak today. Recommendation: RSA-2048. Check tools often already flag 1024-bit keys as warn. Rotating to 2048 bit is bread and butter.

  3. Public key missing (NXDOMAIN)

    The sender signs with selector X, but there's no TXT under X._domainkey.… in DNS. Result: dkim=fail. Common after a provider switch or DNS migration.

  4. Multiple TXT records at the selector

    Two records under the same selector hostname → receivers take the first one or reject both. Exactly one TXT with v=DKIM1 per selector.

  5. No key rotation

    Keys older than 12 months, uncritical in normal operation, but devastating if the key is compromised. Running two selectors in parallel and rotating every six months is common practice.

  6. DNS provider splits long TXT records incorrectly

    2048-bit keys don't fit into a 255-byte string and must be split. Some DNS providers split them incorrectly automatically. Test: dig TXT ….

  7. Custom selector not recognized

    Sender uses custom names like v1, 2024a. Generic check tools only try 10-15 standard names, the custom selector then incorrectly appears as "missing". Manual dig required.

Fix issues, in 4 steps

Identify the active selector

Send a mail to Gmail or Outlook and look at the headers: DKIM-Signature: … d=example.com; s=selector1; …. s= is the active selector.

Rotate the key to 2048 bit

Create a new selector at your sending provider with key length 2048 bit, publish the public key as TXT in DNS, switch the sending system to the new selector.

selector2._domainkey IN TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOC…"

Only delete the old selector after a transition period

Wait 1-2 weeks, during this time mail providers still deliver mail with the old signature from caches. Only then remove the old TXT.

Check DKIM pass in DMARC reports

The aggregate reports show DKIM pass + alignment per IP. If all sources return dkim=pass + aligned, the setup is clean.

Want to check all 8 standards?

The full SelfCheck additionally tests SPF/DKIM/DMARC, BIMI, MTA-STS, TLS-RPT, DNSSEC and MX in a single run.

Start full SelfCheck →

Frequently asked questions about DKIM

What is a DKIM selector?
A DKIM selector is a freely chosen name that addresses the public key in a TXT record. Format: <selector>._domainkey.<domain>. Multiple selectors allow parallel keys for different senders or for key rotation without downtime.
Which key length is secure?
RSA with 2048 bit is the current standard. 1024 bit is considered outdated. Ed25519 is increasingly supported, but useful as a dual setup alongside RSA-2048.
How do I rotate DKIM keys?
Create a new selector with a new key, run it in parallel with the old one, switch sending to the new selector, delete the old selector after a 1-2 week transition period.
My selector has a different name, what now?
The tool check tries the 11 most common names. For custom names it returns warn. Check the selector manually with dig TXT <selector>._domainkey.<domain>.
What does DKIM pass mean?
DKIM pass means: the receiving server cryptographically verified the signature. For DMARC alignment, the d= tag must additionally match the From domain.
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