Email glossary
Know exactly what each signal allows you to conclude.
SMTP acceptance, mailbox delivery, inbox placement and reading are four different states. This glossary connects every term to its operational use in an investigation.
Protocols and actors
08SMTP
Protocol used to submit and transfer email.
IMAP
Protocol for accessing and managing messages stored on a mail server.
POP3
Protocol for retrieving messages from a mail server.
MUA
Mail User Agent: software used to read, compose and organize email.
MSA
Mail Submission Agent: service that accepts a message from a user or application for sending.
MTA
Mail Transfer Agent: server that relays email toward another mail system.
MDA
Mail Delivery Agent: component that deposits a message into a recipient mailbox.
SMTP relay
A service that accepts SMTP traffic and forwards it to other mail servers.
Identity and content
08Envelope sender
SMTP MAIL FROM identity used for transport and bounce handling.
From header
Author address displayed to the recipient in the message header.
Return-Path
Header recording the final reverse-path used for delivery.
Message-ID
Message identifier defined in the Internet Message Format.
MIME
Standards that represent multipart bodies, content types and transfer encodings.
Charset
Character set used to interpret textual content.
Encoding
Representation used to transport content safely, such as base64 or quoted-printable.
Attachment
A MIME body part presented as an attached file.
DNS, authentication and security
09MX
DNS record that identifies mail servers accepting email for a domain.
SPF
DNS-based mechanism authorizing hosts to use a domain in SMTP identities.
DKIM
Cryptographic signature that associates selected message content with a signing domain.
DMARC
Policy and reporting layer built on aligned SPF and DKIM results.
Alignment
Relationship between the visible From domain and an authenticated SPF or DKIM domain.
ARC
Authenticated Received Chain preserves authentication assessments across intermediaries.
BIMI
Specification for publishing a brand indicator for eligible authenticated mail.
TLS
Transport Layer Security encrypts a connection between mail systems.
MTA-STS
Policy mechanism that lets a domain require authenticated TLS for inbound SMTP.
Delivery outcomes
11Acceptance
The next SMTP server returned a successful response for responsibility for the message.
Delivery
The message was delivered to the destination defined by the reporting system.
DSN / NDR
Delivery Status Notification describing delivery success, delay or failure.
Enhanced status code
Three-part status code that classifies mail system outcomes.
Soft Bounce
Industry term for a temporary delivery failure.
Hard Bounce
Industry term for a permanent delivery failure.
Deferral
Temporary refusal that asks the sending system to retry later.
Throttling
Rate limitation applied by a service or recipient.
Greylisting
Technique that temporarily rejects unfamiliar delivery attempts and expects a retry.
Quarantine
Policy action that isolates or diverts a message for review.
Blocking
Policy decision preventing a message or sender from progressing.
Reputation and placement
09Complaint
Recipient signal that a message is unwanted, often through a spam report.
Feedback loop (FBL)
Mechanism through which a mailbox provider returns complaint signals to an eligible sender.
Spam Placement
Classification of an accepted message into a spam or junk folder.
Inbox Placement
Placement of a delivered message in the primary inbox or another desired inbox category.
Suppression
Rule that prevents sending to an address after a bounce, complaint or opt-out.
Reputation
Provider assessment of sending behavior associated with domains, IPs and other signals.
Warm Up
Gradual increase of traffic on a new or changed sending identity.
Shared Ip
Sending IP used by multiple customers or traffic streams.
Dedicated Ip
Sending IP assigned to one customer or controlled traffic set.
Consent and message purpose
06Opt In
A recipient’s affirmative agreement to receive a defined type of communication.
Double Opt In
Subscription process requiring confirmation through a second action.
Opt Out
Action by which a recipient asks not to receive further communications.
Transactional Email
Message primarily triggered by a service transaction or expected account event.
Marketing Email
Message intended to promote products, services or engagement.
One-click unsubscribe
Machine-actionable unsubscribe mechanism using the List-Unsubscribe headers and one-click protocol.