# Email Marketing Glossary: Terms Every Sender Should Know

> Plain-English definitions of the email marketing terms you meet in guides, sending dashboards and campaign reports, from A/B test to warmup.

- Canonical URL: https://acellesend.com/kb/articles/email-marketing-glossary
- Author: Sofia Lindqvist, Email Marketing Editor
- Category: Domain Knowledge — Email marketing fundamentals — consent, deliverability, metrics, list quality, key terms
- Content type: reference · Difficulty: beginner
- Tags: Email Marketing Basics
- Published: 2026-09-30 · Last updated: 2026-10-03
- Reading time: 7 min
- Publisher: AcelleMail Knowledge Base, https://acellesend.com/kb

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## What this covers

A short definition of each term you are likely to meet when you send email campaigns — in this
knowledge base, in your sending provider's dashboard and in campaign reports. The terms are grouped
by topic. Where a term has a full article, the definition links to it.

## Audience and permission

| Term | What it means |
|---|---|
| **List** | A set of contacts you send to. Each contact has at least an email address; most lists also store a name and other details. |
| **Contact (subscriber)** | One person on a list. |
| **Opt-in** | A person agreeing to receive your email — for example by filling in a signup form. |
| **Single opt-in** | The address joins the list as soon as the form is submitted. |
| **Double opt-in (confirmed opt-in)** | After submitting the form, the person clicks a link in a confirmation email before the address joins the list. It proves the address exists and belongs to the person who signed up. |
| **Consent** | The permission a person gave you to email them. Several laws require you to be able to show when and how it was given. |
| **Unsubscribe (opt-out)** | A person's request to stop receiving your email. Every marketing email must offer a working way to do it. |
| **Suppression list** | Addresses you must not send to again — people who unsubscribed, complained or hard-bounced. It is kept even when the address is imported again later. |
| **Segment** | A part of a list chosen by rules, such as "clicked in the last 90 days" or "country is Germany". |
| **Custom field** | Extra information stored for each contact — company, plan, birthday — that you can segment on or insert into an email. |
| **Role address** | An address that belongs to a function rather than a person, such as `info@` or `sales@`. No single person signed up, so consent is hard to show, and some sending providers restrict mail to these addresses. |

## Sending

| Term | What it means |
|---|---|
| **ESP (email service provider)** | A company or platform that sends email for you and gives you tools to manage lists and campaigns. |
| **Sending server (sending service)** | The system that actually transmits your messages: your own mail server, or a service such as Amazon SES, SendGrid or Mailgun. |
| **SMTP** | Simple Mail Transfer Protocol — the standard mail servers use to pass messages to each other. |
| **MTA (mail transfer agent)** | Mail server software that sends and receives email over SMTP, such as Postfix. |
| **Mailbox provider** | The company that hosts the recipient's inbox — Gmail, Outlook.com, Yahoo and so on. |
| **Sending domain** | The domain in your From address, for example `news.example.com`. |
| **Envelope sender (Return-Path)** | The address bounces go back to. It is set when the message is handed over and can differ from the From address the reader sees. See [Return-Path vs Envelope-Sender](https://acellesend.com/kb/articles/return-path-and-envelope-sender-explained). |
| **MX record** | A DNS record naming the servers that accept mail for a domain. |
| **Dedicated IP** | A sending IP address used only by you, so its reputation reflects only your mail. See [Dedicated vs Shared Sending IP](https://acellesend.com/kb/articles/dedicated-vs-shared-ip-address). |
| **Shared IP** | An IP address a sending provider uses for many senders at once. |
| **Throttling** | Limiting how fast you send, for example to a maximum number of messages per hour. See [Email Throttling & Rate Limits](https://acellesend.com/kb/articles/email-throttling-and-rate-limits-explained). |
| **Warmup** | Raising volume gradually on a new IP address or domain, so mailbox providers can build a reputation for it. See [Domain Warmup](https://acellesend.com/kb/articles/domain-warmup). |

## Authentication

| Term | What it means |
|---|---|
| **SPF** | Sender Policy Framework. A DNS record listing the servers allowed to send mail for your domain. |
| **DKIM** | DomainKeys Identified Mail. The sending server signs each message with a private key; the receiver checks the signature against the public key in your DNS. A valid signature shows the message was not changed on the way and was authorised by the signing domain. |
| **DMARC** | A DNS record that tells receivers what to do with mail that fails authentication for your domain — nothing (`p=none`), send it to spam (`p=quarantine`) or refuse it (`p=reject`) — and where to send reports. |
| **Alignment** | For DMARC, the domain that passed SPF or DKIM must match the domain in the From address the reader sees. |
| **BIMI** | A DNS record that lets supporting mailbox providers show your logo next to your mail. It needs a DMARC policy of `quarantine` or `reject`, and Gmail also requires a mark certificate. See [BIMI Setup](https://acellesend.com/kb/articles/bimi-setup-show-your-logo-in-gmail). |

How to publish all three records: [How to Set Up SPF, DKIM, and DMARC Records](https://acellesend.com/kb/articles/how-to-set-up-spf-dkim-and-dmarc-records).

## Delivery results

| Term | What it means |
|---|---|
| **Delivered** | The receiving server accepted the message. It says nothing about which folder the message landed in. |
| **Bounce** | A message the receiving server refused, reported back to the sender. |
| **Hard bounce** | A permanent failure, such as an address that does not exist. Stop sending to it. |
| **Soft bounce** | A temporary failure, such as a full mailbox or a server that is briefly unavailable. The message is retried; an address that keeps soft-bouncing is eventually treated like a hard bounce. See [Hard Bounce vs Soft Bounce](https://acellesend.com/kb/articles/understanding-email-bounce-types). |
| **Complaint (spam report)** | The recipient clicked "Report spam" (or the equivalent) in their mail app. |
| **Feedback loop (FBL)** | A service some mailbox providers run that reports complaints back to the sender, so the address can be suppressed. See [Feedback Loops (FBL)](https://acellesend.com/kb/articles/feedback-loops-fbl-setup). |
| **Inbox placement** | Where a delivered message was filed — the inbox, a tab such as Promotions, or the spam folder. |
| **Deliverability** | Your overall ability to get mail into inboxes, not merely delivered. |
| **Sender reputation** | The trust mailbox providers place in your sending IP addresses and domains, built from how your mail has been received over time. |
| **Spam trap** | An address that no active person uses, kept to catch senders who mail addresses they should not have. |
| **Blocklist** | A list of IP addresses or domains known for sending spam, which many receiving servers check before accepting mail. |

## Messages

| Term | What it means |
|---|---|
| **Campaign** | One email sent to a list or segment at a time you choose, such as a newsletter or a promotion. |
| **Automation (workflow)** | Emails sent automatically when a contact does something or meets a condition, such as signing up or buying. |
| **Transactional email** | A message triggered by one person's action, such as a receipt or a password reset, rather than sent to a list. |
| **Subject line** | The line shown in the inbox before the message is opened. |
| **Preheader** | The short text most inboxes show next to or under the subject line. It is taken from the start of the email unless you set it. |
| **Merge tag** | A placeholder, such as the contact's first name, replaced with each contact's own data when the email is sent. |
| **Call to action (CTA)** | The link or button asking the reader to do the one thing the email is for. |
| **A/B test (split test)** | Sending two versions of an email to parts of a list to see which works better, then sending the winner to the rest. |
| **List-Unsubscribe header** | A hidden header that lets mail apps show their own unsubscribe button. With the "one-click" version (RFC 8058) the app unsubscribes the person without opening a web page. |

## Metrics

| Term | What it means |
|---|---|
| **Open rate** | Unique opens ÷ delivered emails. Opens are detected with a tiny tracking image, so privacy features that load images automatically make the number less reliable. See [Open Rates + Apple MPP](https://acellesend.com/kb/articles/understanding-email-open-rates-in-the-age-of-apple-mpp). |
| **Click rate (CTR)** | Unique clicks ÷ delivered emails. |
| **Click-to-open rate (CTOR)** | Unique clicks ÷ unique opens. |
| **Bounce rate** | Bounces ÷ emails sent. |
| **Unsubscribe rate** | Unsubscribes ÷ delivered emails. |
| **Complaint rate (spam rate)** | Complaints ÷ delivered emails. |
| **Conversion rate** | People who completed the goal — a purchase, a signup — divided by delivered emails or by clickers. Always say which. |

"Unique" means each recipient is counted once, however many times they opened or clicked.

## What to do after

- See how these terms fit together: the other articles in [Domain Knowledge](https://acellesend.com/kb/category/domain-knowledge) explain consent, deliverability, metrics and list quality in depth.
- Read your first report with the definitions at hand: [How to read your email campaign report](https://acellesend.com/kb/articles/how-to-read-your-email-campaign-report).

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## Related articles

- [How Email Marketing Works, From Signup to Sale](https://acellesend.com/kb/articles/how-email-marketing-works): The email marketing cycle in one read: permission, list, message, sending, delivery and measurement, and why each step depends on the one before.
- [Email Deliverability Explained: How Mail Reaches the Inbox](https://acellesend.com/kb/articles/email-deliverability-explained): Delivered is not the same as inboxed. What Gmail, Yahoo and Microsoft weigh before they file your email, their sender rules, and the AcelleMail settings.
- [Email Marketing Metrics Explained: Formulas, Limits and What to Trust](https://acellesend.com/kb/articles/email-marketing-metrics-explained): What each campaign rate divides by, why opens and clicks can mislead, the complaint and bounce limits that matter, and how AcelleMail counts.
- [Email Consent Explained: Opt-In, GDPR, CAN-SPAM and CASL](https://acellesend.com/kb/articles/email-consent-and-opt-in-explained): What makes email consent valid, single vs double opt-in, the proof to keep, and how GDPR, CAN-SPAM and CASL differ, with the AcelleMail settings.
- [Transactional vs Marketing Email: What's the Difference](https://acellesend.com/kb/articles/transactional-vs-marketing-email): Receipts and password resets follow different rules from newsletters and promotions. How to tell them apart, and why to send them separately.