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.
What Resend handles#
Most platforms, including Resend, generate the records or settings you need under a domain authentication or sending settings page, and expose bounce and complaint data in reports. Confirm the exact location in Resend's current documentation.
What you still own#
- 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.
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 Resend set up DKIM signing automatically?
Resend provides the values and some checks, but publishing DNS, aligning domains, and monitoring results remain your responsibility.