Email glossary
Plain-English definitions of the email authentication, deliverability, and sending terms that come up when you run transactional and marketing email.
Email authentication
DKIM (DomainKeys Identified Mail) is an email authentication standard that lets a sending server attach a cryptographic signature to each message, which the receiving server verifies against a public key published in the sending domain’s DNS — proving the message was authorised by the domain and was not altered in transit.
SPF (Sender Policy Framework) is an email authentication standard that lets a domain owner publish, in DNS, the list of mail servers authorised to send email for that domain, so receiving servers can reject or flag mail that comes from any other source.
DMARC (Domain-based Message Authentication, Reporting and Conformance) is an email authentication standard built on top of SPF and DKIM that lets a domain owner tell receiving servers what to do with mail that fails authentication — monitor, quarantine, or reject — and to receive aggregate reports on who is sending mail using their domain.
The return-path, also called the envelope sender or MAIL FROM address, is the address given in the SMTP envelope when a message is sent — separate from the visible From: header — which receives bounce notifications and is the domain SPF checks.
BIMI (Brand Indicators for Message Identification) is a standard that lets a domain with an enforced DMARC policy publish its logo in DNS, so participating mailbox providers can display that logo next to authenticated messages in the inbox.
Deliverability
A hard bounce is a permanent delivery failure — the recipient address does not exist or the domain cannot receive mail — and the address should never be emailed again; a soft bounce is a temporary failure, such as a full mailbox or an overloaded server, where the message may succeed on a later retry.
MTA-STS (SMTP MTA Strict Transport Security, RFC 8461) lets a domain that receives email publish a policy telling sending servers to deliver to it only over TLS with a valid certificate and only to the listed MX hosts, which stops attackers downgrading or redirecting mail in transit.
An email feedback loop (FBL) is a programme run by a mailbox provider that sends the sender a report, usually in Abuse Reporting Format (RFC 5965), whenever one of its users marks a message from that sender as spam, so the sender can stop mailing that person.
Email warmup is the practice of gradually increasing sending volume from a new IP address or domain over several weeks, starting with the most engaged recipients, so mailbox providers can build a positive reputation for it instead of throttling or filtering a sudden burst of mail from an unknown source.
One-click unsubscribe (RFC 8058) is a pair of email headers — List-Unsubscribe with an HTTPS URL and List-Unsubscribe-Post: List-Unsubscribe=One-Click — that lets a mailbox provider unsubscribe a recipient with a single POST request when they click the provider’s own unsubscribe button.