What DKIM signing is#
DKIM (DomainKeys Identified Mail) adds a cryptographic signature header to each message. The public key lives in DNS under a selector, and receivers verify the signature to confirm the message was not altered and was authorized by the signing domain.
Why it matters#
DKIM survives forwarding where SPF breaks, carries domain reputation across sending IPs, and is the most reliable path to DMARC alignment for third-party senders.
How mailbox providers use it#
Gmail, Microsoft, and Yahoo combine authentication results, sender reputation, and recipient engagement into a placement decision made per message. DKIM signing feeds directly into that model, and weaknesses compound with other signals.
How to measure the impact#
- Baseline inbox placement with seed tests before any change.
- Make one change at a time and hold volume steady.
- Re-test after 48 to 72 hours; provider models need time to update.
- Track Postmaster Tools and SNDS alongside your seed results.
Improving it#
- Generate a 2048-bit key pair in your ESP or mail server (1024-bit keys are considered weak).
- Publish the public key as a TXT record at selector._domainkey.yourdomain.com.
- Enable signing in the sending platform and confirm the d= tag in outgoing headers matches your organizational domain.
- Send a test to a seed address and inspect the Authentication-Results header for dkim=pass.
- Schedule key rotation every 6 to 12 months using a second selector so old mail still verifies.
s1._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."Frequently asked questions#
What is a DKIM selector?
A label that lets one domain publish multiple keys. The selector appears in the s= tag and forms the DNS name selector._domainkey.domain.
Why does DKIM pass but DMARC fail?
Alignment. DMARC requires the d= domain to match the From header domain. If your vendor signs with their own domain, set up a custom DKIM domain with them.
Can I use one DKIM key for all my ESPs?
You can publish separate selectors per vendor. Never share private keys between platforms.