Gmail enforces bulk-sender rules: authentication, one-click unsubscribe, and spam-rate thresholds under 0.3% in Postmaster Tools.
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.
Setting up DKIM signing for Gmail#
- 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 Gmail reports results#
Open a delivered test message in Gmail and view the original or headers. Look for Authentication-Results and any provider-specific verdict headers. Use the provider's sender dashboard for aggregate reputation.
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.
Does Gmail require DKIM signing?
Gmail enforces bulk-sender rules: authentication, one-click unsubscribe, and spam-rate thresholds under 0.3% in Postmaster Tools. Treat DKIM signing as required for any meaningful volume.