Email jargon can get dense quickly. This glossary is meant to be:

  • Short enough to skim.
  • Precise enough to be trusted.
  • Linked into deeper guides where needed.

Over time, many of these entries will grow into their own focused pages. For now, this is a single reference page.

IMAP

IMAP – Internet Message Access Protocol

The protocol most modern mail clients use to access mailboxes on a server.

  • Messages live on the server.
  • Clients synchronise folders, flags and message state.
  • Built for multi-device access to the same mailbox.

See also: IMAP Protocol Overview.

POP3

POP3 – Post Office Protocol version 3

An older mailbox access protocol designed for:

  • Downloading messages from a server.
  • Optionally deleting them once retrieved.

It predates always-on connections and multi-device usage. Many POP setups are effectively “download-and-forget.”

SMTP

SMTP – Simple Mail Transfer Protocol

The protocol used to send email between servers and from clients to servers.

  • When you hit “Send” in your mail client, it’s SMTP that hands the message off.
  • IMAP/POP are about mailbox access; SMTP is about transport.

MX Record

MX Record – Mail Exchanger

A DNS record that tells other servers where to deliver mail for a domain.

  • A domain can have multiple MX records with different priorities.
  • If no MX exists, some servers fall back to the domain’s A/AAAA record, but that’s not best practice.

SPF

SPF – Sender Policy Framework

A DNS TXT record that declares which hosts are allowed to send mail for a domain.

  • Helps receiving servers decide if a message’s sending IP is legitimate for that domain.
  • Alone, SPF is not a spam or phishing cure, but it’s a key input.

DKIM

DKIM – DomainKeys Identified Mail

A system where the sending server cryptographically signs outgoing messages with a key that’s published in DNS.

  • Recipients can verify that:
    • The message hasn’t been tampered with in transit.
    • It was authorised by the domain owner (or their provider).

DMARC

DMARC – Domain-based Message Authentication, Reporting and Conformance

A policy layer on top of SPF and DKIM.

  • Tells receivers what to do when SPF/DKIM checks fail (monitor, quarantine, reject).
  • Provides reporting so domain owners can see who is sending mail on their behalf and how often authentication fails.

MTA, MUA, MSA

MTA – Mail Transfer Agent

Software that moves mail between servers (e.g., Postfix, Exim).

MUA – Mail User Agent

The client a human uses to read and send mail (e.g., Thunderbird, Outlook, Apple Mail).

MSA – Mail Submission Agent

The part of the infrastructure that accepts mail from MUAs for delivery, typically authenticated and on a submission port (e.g., 587).

Mailbox, Folder, Label

Mailbox / Folder

In IMAP, a mailbox is a named container for messages (often called a “folder”).

Label

Some providers (notably Gmail) present labels, which can behave like folders but allow a message to appear in multiple “places” at once.

IDLE

IDLE (IMAP IDLE Extension)

An IMAP extension that allows a client to wait for real-time notifications of changes (e.g., new mail) instead of polling.

  • Better for responsiveness and mobile battery life.
  • Not all servers or clients support it; some approximate with short polling intervals.

We’ll expand and refine this glossary as the rest of the site grows.
If you’re using these definitions in your own documentation, you’re welcome to link directly to this page.