DomainKeys Identified Mail, or DKIM, is a technical standard that helps protect email senders and recipients from spam, spoofing, and phishing.  It is a form of email authentication that allows an organization to claim responsibility for a message in a way that can be validated by the recipient.

Specifically, it uses an approach called “public key cryptography” to verify that an email message was sent from an authorized mail server, in order to detect forgery and to prevent delivery of harmful email like spam. It supplements SMTP, the basic protocol used to send email, because it does not itself include any authentication mechanisms.

How does it work?

It works by adding a digital signature to the headers of an email message. That signature can be validated against a public cryptographic key in the organization’s Domain Name System (DNS) records. In general terms, the process works like this:

A domain owner publishes a cryptographic public key as a specially-formatted TXT record in the domain’s overall DNS records.

When a mail message is sent by an outbound mail server, the server generates and attaches a unique DKIM signature header to the message. This header includes two cryptographic hashes, one of specified headers, and one of the message body (or part of it). The header contains information about how the signature was generated.

When an inbound mail server receives an incoming email, it looks up the sender’s public DKIM key in DNS. The inbound server uses this key to decrypt the signature and compare it against a freshly computed version. If the two values match, the message can be proved to authentic and unaltered in transit.

What is a DKIM signature?

A DKIM signature is a header added to email messages. The header contains values that allow a receiving mail server to validate the email message by looking up a sender’s DKIM key and using it to verify the encrypted signature. It looks something like this:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sparkpost.com; s=google; h=from:content-transfer-encoding:subject:message-id:date:to:mime-version; bh=ZkwViLQ8B7I9vFIen3+/FXErUuKv33PmCuZAwpemGco=; b=kF31DkXsbP5bMGzOwivNE4fmMKX5W2/Yq0YqXD4Og1fPT6ViqB35uLxLGGhHv2lqXBWwFhODPVPauUXxRYEpMsuisdU5TgYmbwSJYYrFLFj5ZWTZ7VGgg6/nI1hoPWbzDaL9qh

A DKIM signature header packs in a lot of information, as it is intended for automated processing. As you can see in this example, the header contains a list of tag=value parts. Notable tags include “d=” for the signing domain, “b=” for the actual digital signature, and “bh=” for a hash that can be verified by recalculating using the sender’s public key.

Signatures are by definition unique from message to message, but these basic elements will be present in every DKIM signature header.

How is it related to SPF, DMARC, or other standards?

DKIM, SPF, and DMARC are all standards that enable different aspects of email authentication. They address complementary issues.

• SPF allows senders to define which IP addresses are allowed to send mail for a particular domain.

• DKIM provides an encryption key and digital signature that verifies that an email message was not faked or altered.

• DMARC unifies the SPF and DKIM authentication mechanisms into a common framework and allows domain owners to declare how they would like email from that domain to be handled if it fails an authorization test.

Do I need DKIM?

If you are a business sending commercial or transactional email, you definitely need to implement one or more forms of email authentication to verify that an email is actually from you or your business. Properly configuring email authentication standards is one of the most important steps you can take to improve your deliverability. However, by itself it only goes so far; SparkPost and other email experts recommend also implementing SPF and DMARC to define a more complete email authentication policy.