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 to implement DKIM signing#
- 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..."How to verify it worked#
Send a test message to seed mailboxes at Gmail, Outlook, and Yahoo, then inspect the Authentication-Results and delivery headers. Repeat after any DNS or sending-platform change.
Common mistakes#
- Signing with the ESP's domain (d=esp.com) instead of yours, which passes DKIM but fails DMARC alignment.
- Copying the public key with line breaks or quotes broken, producing a permanent verification failure.
- Modifying message bodies after signing (footers injected by gateways) which invalidates the signature.
- Never rotating keys, leaving a compromised key valid indefinitely.
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.