DKIM Domain (d=)

What is a DKIM Domain (d=)?

The DKIM Domain, represented by the d= tag in a DKIM signature, identifies the domain that takes responsibility for an email message. This domain appears in the DKIM-Signature header and is used by receiving mail servers to verify message authenticity. It is one of the most important fields in DKIM because it defines the entity vouching for the message’s integrity.

When a sender’s mail server signs a message using DomainKeys Identified Mail (DKIM), the signature includes the d= value, the selector (s=), and the cryptographic hash of the message. The receiving server retrieves the public key for that domain from DNS, using the selector to locate the correct record, and validates the signature. If verification succeeds, the recipient can trust that the message was not altered after signing and that it originated from the stated domain.

How the DKIM Domain (d=) Tag Works

Each outgoing message contains a DKIM-Signature header like the following:

Here, d=example.com tells the verifier that the domain example.com is responsible for the signature. The receiving server looks up the public key in DNS at:

If the public key retrieved from DNS matches the private key that signed the message, the DKIM check passes. The d= value may represent the organization’s main domain or a subdomain dedicated to a particular mail stream or provider.

Role of the DKIM Domain in Authentication

The DKIM Domain is central to how receivers determine domain alignment under DMARC. When DMARC evaluates a message, it checks whether the domain in the DKIM signature (d=) matches or is a subdomain of the visible From domain. If they align, and the DKIM signature is valid, the message passes DKIM alignment under DMARC.

Common implementation practices include:

• Using a single signing domain (such as d=example.com) for all outgoing mail
• Using delegated subdomains (such as d=mail.example.com) for specific vendors or systems
• Ensuring each signing domain publishes a valid DKIM public key in DNS
• Monitoring DKIM results in DMARC reports to verify consistent alignment

Without proper configuration, mismatched domains can cause DKIM to pass but DMARC to fail. For example, if your DKIM signature uses d=vendor.com while your From domain is example.com, the message may not align under DMARC policy.

DKIM Domain (d=) and DMARCeye

DMARCeye analyzes DKIM domains across all authenticated traffic to reveal which systems and vendors are signing messages on your behalf. By mapping each d= value to its organizational domain, DMARCeye shows alignment status, key validity, and any mismatched or untrusted signers.

The platform also tracks signature success rates over time and alerts you to missing or expired DKIM keys. With this visibility, organizations can ensure consistent authentication across all senders, maintain policy alignment, and protect brand integrity against spoofing attempts.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.