Email Glossary

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.

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.

Transactional email is a message sent to a single recipient in response to an action they took — a password reset, receipt, or shipping notification — whereas marketing email is sent in bulk to a list to promote or inform, and the distinction matters for consent law, deliverability, and which sending infrastructure you should use.