Email Authentication Glossary
Concise, technically accurate definitions for the email authentication terms that matter — protocols, alignment rules, threat patterns.
ARC
A protocol that lets intermediate mail servers preserve the original authentication results of a message as it is forwarded, so the final receiver can still trust them.
Read definitionBIMI
A DNS standard that lets a domain owner publish a logo to display next to authenticated messages in supporting mail clients.
Read definitionBusiness Email Compromise (BEC)
A targeted email fraud attack that impersonates a trusted internal or external party — typically an executive, vendor, or supplier — to trick the recipient into sending money or data.
Read definitionDKIM
A cryptographic signature attached to outbound email that proves the message was authorised by the domain owner and was not modified in transit.
Read definitionDKIM Selector
An arbitrary label that lets a domain publish multiple DKIM keys at once, each at a separate DNS hostname.
Read definitionDMARC
An email authentication policy framework that uses SPF and DKIM to detect and prevent domain spoofing, and reports back to domain owners on who is sending under their name.
Read definitionDMARC Alignment
The check that requires the SPF or DKIM domain to match the visible From header domain in order for DMARC to pass.
Read definitionDMARC Policy
The action a domain owner asks receivers to take when a message fails DMARC: monitor (none), quarantine, or reject.
Read definitionEmail Spoofing
Sending an email message with a forged From header that makes it appear to come from a domain the sender does not control.
Read definitionMTA-STS
A standard that lets a domain owner declare that incoming mail must be delivered over TLS, with policies hosted over HTTPS rather than DNS.
Read definitionRUA (Aggregate Report)
A daily XML summary report sent by mail receivers to a DMARC-publishing domain, listing every IP that sent under the domain and how each authenticated.
Read definitionSPF
A DNS-published list of IP addresses and hostnames authorised to send email on behalf of a domain.
Read definition