Glossary
DKIM
Also called DomainKeys Identified Mail.
Last reviewed 11 October 2026
What is DKIM?
DKIM is a cryptographic signature over a message’s headers and body, made with a private key held by whoever sends the mail and checked by the receiver against a public key published in DNS under a selector, and it is defined in RFC 6376.
It survives forwarding, which is exactly where SPF fails, because the signature travels with the message rather than describing the route it took. The two are complementary rather than alternatives, and the bulk sender requirements ask for both.
The signature names a domain of its own, and that domain is what accumulates reputation. If your provider signs with its own domain rather than yours, the reputation being built belongs to the provider and to every other sender in its pool.
A DKIM key cannot be discovered. The public key sits at selector._domainkey under your domain and the selector is an arbitrary string chosen by whoever set it up, so there is no way to enumerate the keys on a domain; any signed message you have received names the one in use in the s= tag of its DKIM-Signature header.
See also
- SPF
SPF is a DNS record listing the servers allowed to send mail for a domain, checked by the receiver against the envelope sender rather than against the From line a reader sees, and it is defined in RFC 7208.
- DMARC
DMARC is the DNS policy that ties SPF and DKIM to the domain a reader actually sees, telling receivers what to do with mail from your domain that aligns with neither and asking for aggregate reports about it, and it is defined in RFC 7489.
- Alignment
Alignment is the DMARC requirement that the domain a reader sees in the From header match a domain that passed SPF or DKIM, which is why a message can pass both of those checks and still fail DMARC.
- Sender reputation
Sender reputation is what a mailbox provider has learned about the domain and the infrastructure your mail comes from, assembled from complaints, bounces, spam trap hits and how the people who receive your mail behave, and it is what decides the folder.
Read next
This term is covered properly in Email deliverability for small senders: what decides it, which is a longer piece on the subject.
Every other term is on the glossary index , the longer writing is in the guides, and what this site itself does is on the home page.
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