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The Ultimate Email Marketing Glossary

 If you’re ever stumped by a new email marketing term, don’t panic! Browse our glossary to find out what it means.



A list of contacts that the user deems acceptable to receive email from; should not be filtered or sent to the trash or spam folder.


Apple Mail Privacy Protection (MPP)

The policy allows Apple Mail users to prevent senders from using tracking pixels to measure their open rates and device usage while also masking their IP addresses to prevent location tracking.



The process of attempting to verify the digital identity of a sender. There are three main types of email authentication: SPF, DKIM, and DMARC. SPF and DKIM are the most widely used and accepted forms of email authentication.



A list of IP addresses that have been reported and listed as “known” sources of spam. Blocklists (formerly known as blacklists) are often used by mailbox providers to help decide whether to deliver an email to the inbox or filter it to the spam folder.



Brand Indicators for Message Identification (BIMI) is a small icon or “indicator” next to the name of the sender associated with an email in the inbox, often a company logo. Implementing BIMI requires an enforced DMARC policy.


Contact Verification

Identifying contact information that is invalid before you contact them. Mailing to invalid email addresses increases bounce rates, which damages your sender reputation.


Customer Data Platform (CDP)

Software that unites customer data from different sources in one location. This can include behavioral data, transactional data, and demographic data. Common CDPs include Segment, Bloomreach, and Emarsys.


Click-Through Rate (CTR)

The ratio of unique clicks on a link(s) within an email to the total number of recipients of the email, typically expressed as a percentage. The click-through rate measures direct responses only. It does not consider people who later visited a website in response to an email marketing campaign.


Click-to-Open Rate (CTOR)

Of people who opened your email, the percentage of them that clicked a link. Once an effective measure of engagement, it has become less reliable after Apple MPP has falsely inflated open rates.


Customer Relationship Management (CRM)

System that tracks and manages a company’s relationships with their current and prospective customers in one database. Common CRMs include Salesforce, HubSpot, and Microsoft Dynamics 365.



Refers to the percentage of emails that reached the subscriber’s inbox. Tabs like primary, promotional, and updates are all part of the inbox and are treated as such when measuring deliverability.



Common metric found in ESPs that refers to the number of emails that reached the subscriber’s inbox OR their spam folder. Typically thought of as the Total Number of Emails Sent minus Bounced Emails.



Domain-based Message Authentication, Reporting & Conformance is a technical specification created by organizations to reduce email phishing and fraud. It is universally recommended to email marketers as it is key in both protecting email from malicious activity and improving deliverability.



DomainKeys Identified Mail lets an organization take responsibility for a message while it is in transit. The organization is a handler of the message, and their reputation is the basis for evaluating where it should be delivered. When working in tandem with SPF, it protects your brand from spoofing and ensures emails sent to your subscribers were actually sent from you.



A particular organization’s registered name on the Internet (e.g.,


Email Service Provider (ESP)

Another name for an email broadcast service provider, a company that sends email on behalf of their clients. Popular ESPs are HubSpot, Mailchimp, Marketo, and Salesforce Marketing Cloud.


Email Engagement

The degree to which your subscribers interacted with your email. This includes actions like opening your email and clicking through to your website.


Feedback Loop (FBL)

A mechanism, process, and signal that are looped back to control a system within itself. For example, mailbox providers offer FBL programs to senders that wish to remove users that complain about email they receive via the “This is Junk/Spam” button.



A technique used by some mailbox providers and email receivers to thwart spammers. A receiving mail server using greylisting will temporarily reject any email from a sender it does not recognize. The receiver presumes that if the sender is legitimate, the originating server will most likely try again to send it later at which time the receiver will accept it. Greylisting presumes that if the sender is a spammer, they will not retry later to transmit their message. Greylisting has disadvantages and is somewhat controversial.


Hard Bounce

Message sent to an invalid, closed, or nonexistent email account. Typically, hard bounced emails can be identified with a 500 series SMTP reply code.



Also referred to as “non-responders.” Defined as the email recipients who have not taken any action on your emails (opens, clicks) in a certain amount of time.


Inbox Placement Rate (IPR)

The rate of emails that were delivered to the inbox, versus the spam folder and missing. Calculated as Number of Emails Delivered to the Inbox divided by Total Number of Emails Sent.


Internet Service Provider (ISP)

A company that provides access to the internet to consumers. Popular ISPs include AT&T, Verizon, and Xfinity.


IP Address

A unique number assigned to each device connected to the Internet. An IP address can be dynamic, meaning it changes each time an email message or campaign goes out, or it can be static, meaning it does not change. Static IP addresses are best, because dynamic IP addresses often trigger spam filters.


List Fatigue

List fatigue happens when a mailing list’s members are sent too many offers (or too many of the same offers) in too short a period of time, causing them to be less engaged and thus harming your overall results.


List Hygiene

The act of maintaining a mailing list so that hard bounces and unsubscribed names are removed from mailings. Some list owners also use an Email Change of Address (ECOA) service to update old or abandoned email addresses (hopefully with a permission step baked in) as part of this process.



The List-Unsubscribe header is text you can include in the header portion of your messages, allowing recipients to see an unsubscribe button they can click if they would like to automatically stop future messages. List-Unsubscribe is currently being used by Gmail,, and Cloudmark.


Mailbox Provider (MBP)

Mailbox providers are a provider of email hosting. They implement servers that send, receive, accept, and store emails. Common mailbox providers include Gmail, Outlook, and Yahoo!.



When using seed lists for deliverability testing, missing results mean that the seed address did not receive the message in the Inbox nor Spam folder. It is possible that your message was blocked or was not properly sent.


Open Rate

The percentage of emails opened of the total amount sent, not accounting for what number of those were actually delivered to the inbox. The open rate used to be a key metric for judging a campaign’s success, but is less reliable after Apple MPP in which proxy opens from Apple can be falsely calculated as actual opens. This has resulted in a global increase of open rates by 50%.



Opt-in email marketing means sending marketing messages only to people who explicitly requested them. If a customer asks for a specific piece of information, you have the permission to send that information and nothing more. To continue sending marketing emails you need the explicit permission to do so (“Please send me announcements and special offers via email”, for example).



Email marketing that assumes a general permission to send marketing messages to everyone who has not explicitly stated that they do not want to receive such information. Spammers operate on this highly problematic premise. Opt-in email marketing, where messages are only sent to those who request them, is much more effective.



A form of identity theft in which a scammer uses an authentic-looking email to trick recipients into giving out sensitive personal information, such as credit-card or bank account numbers, Social Security numbers, and other personal identifiable information (PII).


Pristine Spam Traps

Email addresses created solely to capture spammers (sometimes referred to as honey pots). These email addresses were never owned by a real person, do not subscribe to email programs and of course will not make purchases. Many spam trap operators will post pristine traps across the Internet. They are usually hidden in the background code of webpages and are acquired by a spambot scraping email addresses. If you’re hitting pristine traps this typically indicates you have a bad data partner.


Recycled Spam Traps

Email addresses that were once used by a real person. These email addresses are abandoned email accounts that are recycled by mailbox providers as spam traps. Before turning an abandoned email address into a spam trap, mailbox providers will return unknown user error codes for a year. Once Mailbox Providers reactivate (recycle) the abandoned email address, mail is once again allowed to be received by the email address. If you’re hitting recycled spam traps this typically indicates you have poor data hygiene.



The ability to slice a list into specific pieces determined by various attributes, such as open history or opt-in source.


Sender Reputation

Sender reputation is comprised of domain and IP reputation, and is developed using a variety of metrics, including complaint rate, unknown user rate, volume, and spam trap hits. Mailbox providers consider a sender’s reputation when determining inbox vs. spam placement of emails. A sender’s reputation can be tracked using our Sender Score tool.


Seed List

A collection of email addresses you include in your sends to test if your campaigns are reaching the inbox. Each email address is called a seed.


Soft Bounce

Email sent to an active (live) email address that is turned away before being delivered. Often, the problem is temporary, for example, the server is down or the recipient’s mailbox is over quota. The email might be held at the recipient’s server and delivered later, or the sender’s email program may attempt to deliver it again. Typically, soft bounced emails can be identified with a 400 series SMTP reply code.



Spam is an email message that you did not ask for and do not want from somebody you do not know. Not all unsolicited emails are spam, however, nor are all emails that land in the spam folder actually spam.


Spam Filter

A mechanism used to identify spam email and keep it out of the recipient’s inbox.


Spam Trap

Getting caught in one can cause you to be blocklisted, your emails to be automatically sent to the spam folder, and your sender reputation to be damaged. The three types of spam traps are typo traps, recycled traps, and pristine traps.


Sender Policy Framework (SPF)

A protocol used to eliminate email forgeries. A line of code called an SPF record is placed in a sender’s DNS information. The incoming mail server can verify a sender by reading the SPF record before allowing a message through.



The practice of changing the sender’s name in an email message so that it looks as if it came from another address.



The process of joining a mailing list, either through an email command, by filling out a web form, or offline by filling out a form or requesting to be added verbally. A subscriber is the person who has specifically requested to join a mailing list.


Sunsetting Policy

The practice of having some process in place for removing unengaged addresses from your active mailing list.



The practice of regulating how many email messages a broadcaster sends to one mailbox provider or mail server at a time. Some mailbox providers bounce email if they receive too many messages.


Typo Spam Trap

An email address with a misspelled domain name (such as or used by mailbox providers and blocklist providers to identify senders with poor list acquisition and list hygiene practices.

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