Email Deliverability Glossary

101 Terms You Should Know

As with any industry, marketers tend to throw out a lot of jargon, and if you’re new to the wonderful world of email, much of it may be gibberish to you. To help the newbies and veterans alike, we’ve compiled this list of the essential email deliverability terms you should know.



Authentication is the process of attempting to verify the digital identity of the sender of a communication. In email marketing, there are three main types of authentication: SPF, DKIM, and DMARC.



Lists of IP addresses that have been reported and listed as known sources of spam. There are public and private blacklists. Public blacklists are published and made available to the public, either as a free service or for a fee. There are hundreds of well-known public blacklists. For more information about blacklists, check out the Blacklists: The Ultimate Guide infographic.


Many mailbox providers block email from IP addresses or domains that have been reported to send spam or viruses, or have content that violates email policy or spam filters.

Bounce Rate

Bounced email is the opposite of delivered email. These are the messages that fail to get delivered for any reason. Bounced mail can be further categorized into hard bounces and soft bounces. For more information about bounce rates, read The Email Marketer’s Guide to Bounce Processing.

Bulk Folder

Also referred to as “Junk” or “Spam” folder in some email clients.



Popular name for the US law regulating commercial email. The full name for this law is the Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003.


Also known as “whitelisting,” certification is a method of accrediting good senders in order to achieve better inbox placement. Messages from certified senders typically receive less rigorous filtering by mailbox providers. For more information about Certification, download the Certification fact sheet.

Click-Through Rate

Calculated by dividing clicks by the volume of email delivered. This metric is commonly used to measure email engagement, but it is actually far less useful than click-to-open rate. For more information about click-through rates, check out the Guide to Email Marketing Metrics.

Click-to-Open Rate

Measured by calculating the ratio of total clicks to total opens. Click-to-open is the best and most accurate of the click-based metrics, and provides valuable insight into the effectiveness of your email content and design.


A spam filter company that uses a network of users as a feedback mechanism to identify and block spam. Their Global Threat Network is fed by various means but most notably through their desktop spam filter and through “This is Spam” buttons that mailbox providers contribute through their Cloudmark Authority product. For more information about Cloudmark, check out this blog post.

Commercial Message

A message that advertises or promotes a commercial product or service, including content on a website operated for commercial purposes. According to CAN-SPAM, these message are required to have a US Postal Service address and an unsubscribe link.

Complaint Rate

Calculated by dividing the number of spam complaints by the number of emails delivered. Recipients of an email can complain using the “This is Junk/Spam” button in their email client. Complaints are a strong indicator of negative engagement and this metric is useful for identifying patterns and sources of complaints, but may be distorted by deliverability issues. For more information about complaint rates, read The Hidden Metrics of Email Deliverability.


Content is a critical component of the filtering process. Content analysis technology scans every part of an email, including the header, footer, code, HTML markup, images, text color, timestamp, URLs, subject line, text-to-image ratio, language, attachments, and more.

Content Filters

Software filters that block email based on text, words, phrases, or header information within the email itself. The goal is to identify spam and filter to the Bulk or Junk mail folders, although this often results in “false positives.” For more information about content filters, read The Ultimate Guide to Email Deliverability.

Conversion Rate

Calculated by dividing the number of conversions by the number of visits. Although a strong indicator of subscriber engagement, this metric typically speaks more to the quality of landing page or website content than email content.


Dedicated IP Address

An IP address that is used by a single sender or company. This is desirable because no other marketer or company is sending email from this IP address, putting you in control of your domain reputation.

Deleted Without Reading Rate

Measures how often a recipient deletes email without reading it. This metric provides powerful insight into the difference between subscribers who do not want to read your email and those who don’t open your email because they check their email infrequently. For more information about deleted without reading rates, read The Hidden Metrics of Email Deliverability.


Refers to the entire subject area of getting your emails delivered to the right place. Deliverability involves much more than just your delivery rate. To learn more about deliverability, check out the 2015 Deliverability Benchmark Report.

Delivery Rate

Calculated by dividing the volume of emails delivered by the volume of emails sent. Note: “delivered” doesn’t necessarily mean your email hit the inbox—just that it wasn’t bounced or rejected.


A protocol that allows an organization to take responsibility for transmitting a message in a way that can be verified by mailbox provider. This verification is made possible through cryptographic authentication. For more information about DKIM, check out this blog post.


Domain-based Message Authentication, Reporting & Conformance (DMARC) is an email authentication protocol that ensures legitimate email is properly authenticating against established DKIM and SPF standards, and that fraudulent activity appearing to come from domains under the organization’s control (active sending domains, non-sending domains, and defensively registered domains) is blocked.


A particular organization’s registered name on the internet (i.e.,

Domain Name System (DNS)

A naming system for computers, services, or other resources connected to the internet. DNS turns user-friendly domain names like into a corresponding IP address such as

Double Opt-in

A technique for reducing spam. Double opt-in requires that a subscriber not only sign up for an email via a web form, but also confirm their subscription by responding to a confirmation email.


Email Client

A program used to read and send email messages. Unlike an email server, which transports mail, an email client is the interface used by the end user to access their email account. Email clients can be software application like Outlook and Lotus Notes or webmail services like the ones provided by Yahoo and Gmail.

Email Service Provider (ESP)

An email service provider (ESP) is a company that handles email sending for its clients. Many ESPs also offer email marketing services, often as part of a marketing cloud. Examples of email service providers include MailChimp, Responsys, and Salesforce Marketing Cloud (formerly ExactTarget).


A measurement of how involved subscribers are with your email program. Engaged subscribers are those who want to receive your emails, open them regularly, and interact with the content. For more information about engagement, check out this blog post.

Engagement Metrics

Engagement-based filtering is one way mailbox providers determine whether or not an email is spam. The metrics used in engagement-based filtering include things like messages read, messages replied to, messages forwarded, messages moved to other folders, and messages marked as “not spam.”

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A technology that helps mailbox providers make filtering decisions about email content. Fingerprints are very small hashes or “checksums” of content. Once these fingerprints are created and stored, they can be compared with the fingerprints of confirmed spam.

Feedback Loop

A service offered by mailbox providers that offers insight into complaints. For mailbox providers, a feedback loop provides signals from subscribers to determine what is wanted email versus unwanted. For senders, feedback loops provide a way to identify and remove subscribers who complain about email they receive via the “This is Junk/Spam” button. Senders must sign up for feedback loops with each mailbox provider that offers this service. For more information about feedback loops, check out this blog post.

Forwarded Rate

Measures how frequently subscribers forward your email on to others. This metric is useful to gauge the virality of your content, and a high forwarded rate indicates strong subscriber engagement. For more information about forwarded rates, read The Hidden Metrics of Email Deliverability.

From Line

Indicates the sender of an email. Typically comprised of a “Friendly From Address” and a “Friendly From Name,” which is usually the sender’s or company’s name.



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


Email that the recipient has requested and is therefore not unsolicited (like spam), but is often not read by the recipient for a long period of time (if at all). Graymail has been described as “email you want, but not right now.” Graymail differs from spam in that the recipient has signed up to receive it. The name “graymail” is meant to convey the idea that classifying such email as spam or not spam isn’t black and white.


Hard Bounce

Hard bounces are messages that are permanently rejected, typically due to issues with list quality. Messages sent to an invalid, closed, or nonexistent email account will receive a hard bounce. Typically, hard bounced emails can be identified with a 500 series SMTP reply code.

HTML Message

Email message which contains Hyper Text Markup Language syntax and encoding. HTML messages must be properly encoded and receiving email clients must be capable of rendering HTML. Senders often utilize HTML in email messages to take advantage of text formatting, images, and design layout beyond what is possible with plain text messages and encoding.



Spammers often create emails as a single large image, in an effort to bypass spam filters. Embedding large images can also slow the email server’s ability to process mail. As a result, spam filters will often flag such emails as spam and stop delivery, so it’s important to maintain a balance text and imagery in your emails. Keep in mind also that some mailbox providers turn off images by default, so it’s likely images won’t be seen anyway.

Inactive Subscribers

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

Measures the percentage of sent email that actually lands in the subscribers’ inbox—a far more accurate measure than delivery rate. Calculated as number of emails delivered to the inbox divided by total number of emails sent. For more information about inbox placement rates, read the Deliverability Benchmark Report.


Refers to the actual hardware used to deploy your emails or have your emails deployed on your behalf by an email service provider (ESP). The hardware is commonly referred to as your mail transfer agent (MTA).

Internet Service Provider (ISP)

A company in the business of providing internet access to consumers and business. Examples include AT&T U-verse and Comcast Xfinity.

IP Address

An exclusive number all information technology devices (printers, routers, modems, et al) use which identifies and allows them the ability to communicate with each other on a computer network. IP addresses can also be dedicated or shared. For more information about IP addresses, check out this blog post.

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List Fatigue

A condition producing diminishing returns from a mailing list whose members are sent too many offers, or too many of the same offers, in too short a period of time.

List Hygiene

The act of maintaining a list so that hard bounces and unsubscribed names are removed from mailings.

List Purchase

The process by which a publisher or advertiser pays a list owner for full access to their email list. The publisher or advertiser would then own the list and send to it over their own system. This practice is typically frowned upon and can lead to high complaints and spam trap hits, as purchased lists are usually of poor quality.

List Rental

The process by which a publisher or advertiser pays a list owner to send its messages to that list. Usually involves the list owner sending the messages on the advertiser’s behalf, and the publisher or advertiser never gains full access to the list unless those subscribers specifically opt-in to their email program. List rentals can be successful when highly targeted.


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.


Mail Transfer Agent (MTA)

A mail transfer agent is a server application that accepts email messages for relay or delivery to local recipients. MTAs are programs on mail servers that are responsible for routing and sometimes delivering mail.

MX Record (Mail Exchange Record)

An MX record is a type of resource record in the Domain Name System (DNS) specifying how internet email should be routed using the Simple Mail Transfer Protocol (SMTP).

Mobile Optimized

The process of adjusting email content and design for the mobile user. Key aspects of mobile optimization include reducing visuals to increase download speed, streamlining design, and increasing the size of links and CTAs.


Open Rate

Calculated by dividing the number of emails opened by the number of emails delivered. This metric is actually less useful than you’d think, because an email will not register as “opened” unless images are displayed in the message—either through settings or active loading. Open rate is similar to read rate. For more information about open rates, read the Guide to Email Marketing Metrics.


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.

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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). To learn more about phishing, check out The Marketer’s Guide to Email Fraud.

POP (Post Office Protocol)

A protocol that defines an email server and a way to retrieve mail from it. Incoming messages are stored at a POP server until the user logs in and downloads the messages to their computer. While SMTP is used to transfer email messages from server to server, POP is used to collect mail with an email client from a server.


The person who manages mail servers at an organization. Usually the one to contact at a particular server/site to get help, information, or to log complaints.

Preview Pane

A setting that desktop and webmail email clients offer that allow users to preview content without actually clicking on the message.

Pristine Spam Traps

Also called “honey pots,” pristine spam traps are created solely to capture bad mailers. 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 (seed) pristine traps across the internet on various participating websites. 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.


Read Rate

The percentage of email recipients who have marked your email as “read” in their email client. Read rate is similar to open rate, but it is far more accurate because it accounts for all emails viewed, regardless of image rendering. For more information about read rates, check out The Hidden Metrics of Email Deliverability.

Recycled Spam Traps

These email addresses once belonged to a real person, but were converted to spam traps after being abandoned. Before turning an abandoned email address into a spam trap, mailbox providers will return unknown user codes for a year. Recycled spam traps identify legitimate senders with weak list hygiene and data quality practices. For more information on recycled spam traps, check out this blog post.

Re-Engagement Campaign

An email campaign sent to inactive subscribers in an attempt to win them back and get them to engage with your emails again. A re-engagement campaign can be structured as a single email, or an email series. For more information on re-engagement, read the Email Win-Back Report.

Rejected Rate

Rejected email is a subset of bounced email and includes only those messages that fail to get delivered due to reputation issues (e.g., complaints, spam traps, blacklisting). For more information about rejected rates, read the Guide to Email Marketing Metrics.

Reply-to Address

The email address that receives messages sent from users who click “reply” in their email clients. Can differ from the “from” address, which may be an automated or unmonitored email address used only to send messages to a distribution list. “Reply-to” should always be a monitored address.


Sender reputation is comprised of domain and IP address reputation, and is calculated using using algorithms and heuristics, leveraging millions of data points and hundreds of parameters including complaint rate, unknown user rate, volume, and spam trap hits. Mailbox providers consider a sender’s reputation when determining inbox vs. junk placement of emails. A sender’s reputation can be tracked using Return Path’s Sender Score tool. For more information about reputation, read the 2016 Sender Score Benchmark Report.

Responsive Design

Responsive design allows your email to automatically re-format and re-size to optimize for whatever screen size your recipient is using to read your email. It can also be used to hide non-essential elements of the email from the mobile reader, thus making sure the main CTA (call to action) of the email is easily found. It may also change various other elements of the email, including text size and color, background images, and background color.


Also referred to as the “bounce address” or “envelope sender address,” this is the address a message really came from, as opposed to the “friendly from” address. It is also the address to which any undeliverable message notices (bounces) are sent. This term should not be confused with the awesome company of the same name, which doesn’t have a hyphen.

Reverse DNS (rDNS)

The process by which an IP address is matched correctly to a domain name, instead of a domain name being matched to an IP address. Reverse DNS is a popular method for catching spammers who use invalid IP addresses. If a spam filter or program can’t match the IP address to the domain name, it can reject the email.


Seed List

A seed list is a list of email addresses that is used to test emails across various email clients and devices. Typically, an email marketer will send a new email to the seed list before sending it out to the rest of their list.


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


This is a generic term that refers to any company sending email to a large number of subscribers.

Sender Score

A proprietary Return Path website used to evaluate sender reputation. Ranging from 0 to 100, the Sender Score is an indication of the trustworthiness of an email sender’s IP address and is used by mailbox providers and filters to determine additional email filtering criteria. For more information about Sender Score, read the 2016 Sender Score Benchmark Report.


A program or computer system that stores and distributes email from one mailbox to another, or relays email from one server to another in a network.

Shared IP Address

An IP address used by multiple marketers or companies to deploy email. Because the overall reputation for that IP address is based on all mail deployed from it, the IP address reputation cannot be managed by an individual sender. As a result, all senders are negatively affected if even one sender on the shared IP address is sending spam. For more information about shared IP addresses, check out this blog post.

Simple Mail Transfer Protocol (SMTP)

The main protocol used to send email over IP networks, defined by RFC 821 and RFC5321.

Smart Network Data Services (SNDS)

Offered by, SNDS provides data to senders based on actual mail sent to subscribers, to help senders understand and improve their reputation at SNDS gives senders access to detailed complaint and filtering data about individual IP addresses, and it also includes a junk email reporting program, which lets senders receive reports when users send their messages to the spam folder.

Soft Bounce

Soft bounces are messages that are temporarily rejected, often due to issues with the recipient’s mailbox or server (e.g., mailbox full or server down). 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 bounces can be identified with a 400 series SMTP reply code.


Spam is unsolicited email. Not all unsolicited email is spam, however. Most spam is sent in bulk to a large number of email addresses and advertises some product. Spam is an email message that you did not ask for and do not want from somebody you do not know, who wants to sell you something. To learn more about spam, read The Ultimate Guide to Email Deliverability.


An email spam reporting service, once privately owned but now a division of Cisco Systems. Many mailbox providers check the IP addresses of incoming email against SpamCop’s records to determine whether the address has been blacklisted due to spam complaints.

Spam Filter

A mechanism used to identify spam email and keep it out of the recipient’s inbox. Originally, filters were designed primarily to identify spam and block it or place it in the spam folder. Today, some mailbox providers use filters to categorize messages for inbox-organization purposes (e.g., social media and newsletters).

Spam Folder Placement Rate

The rate of emails that are marked as junk/spam by recipients, typically expressed as a percentage over total number of emails delivered. For more information about spam folder placement rates, read The Hidden Metrics of Email Deliverability.

Spam Trap

Spam traps are email addresses that don’t belong to active users and are used to identify both spammers and senders with poor data quality practices. Mailbox providers, filtering companies, and blacklist administrators create and manage spam trap networks to monitor email received at these addresses. Spam traps can be pristine or recycled.


An international organization based in London and Geneva that tracks email spammers and spam-related activities. The Spamhaus project is responsible for compiling several widely used anti-spam lists. Many mailbox providers use the lists to reduce the amount of spam that reaches their users. For more information about Spamhaus, check out this blog post.

Sender Policy Framework (SPF)

An email authentication protocol that allows the owner of a domain to specify which mail servers they use to send mail from that domain. For more information about SPF, check out this blog post.

Spear Phishing

Spear phishing is a fraudulent email tactic that targets a specific organization, seeking unauthorized access to confidential data, often through an email that appears to come from someone in a position of authority within that organization. Spear phishing attacks are typically targeted, not random. For more information about spear phishing, check out this blog post.


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

Sender Reputation Data (SRD)

Used by, Microsoft Live Hotmail, and MSN Hotmail, SRD is a collection of non-biased responses from feedback loop participants over time. Along with other sources of reputation data such as the Junk Email Reporting Program (JMRP), the Windows Live Sender Reputation Data helps to train and improve the way Microsoft’s SmartScreen technology properly classifies messages based on email content and sender reputation. For more information about SRD, check out this blog post.


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.


The person who has specifically requested to join a mailing list.

Suppression File

A list of email addresses you have removed from your regular mailing lists, either because they have opted out of your lists or because they have notified other mailers that they do not want to receive mailings from your company. CAN-SPAM requires all senders to maintain a suppression list.


This is Not Spam (TINS) Rate

Measures how frequently recipients click on the “This is not spam” button after an email is delivered to the spam folder. This metric is a powerful indicator of subscriber engagement. For more information about TINS rates, read The Hidden Metrics of Email Deliverability.


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

Transactional Mail

Transactional messages are defined under CAN-SPAM as any email “facilitating, completing, or confirming a previously agreed upon transaction.” Unlike commercial messages, transactional messages aren’t required to have a US Postal Service address or an unsubscribe link.

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Unknown User

An unknown user is an email address that never existed, has been terminated by the mailbox provider, or was abandoned by the end user. Mailbox providers will return a hard bounce code indicating when email is sent to an unknown user.

Unsubscribe Rate

Calculated by dividing the number of unsubscribes by the number of emails delivered. Be cautious of using this metric in isolation, as a declining unsubscribe rate can be caused by a variety of things, such as improving engagement or a decreasing inbox placement rate—two very different situations.



Volume is the amount of email you send. It’s important to maintain a relatively consistent volume of email over time, as dips and spikes in volume is often interpreted by mailbox providers as potentially spammy behavior. To learn more about volume, check out the State of the Inbox 2014.


Warm-up Period

When switching to a new IP address, it’s critical to start slowly with low send volume, in order to establish a good sender reputation with the new IP address. This gradual ramp-up is known as the warm-up period. It may take a month or more to reach your normal send volume on the new IP address, but this will help to avoid having your mail blocked. For more information about the warm-up period, check out this blog post.

Welcome Message

A welcome message can help to reduce spam complaints from new subscribers who don’t recognize your brand. A timely welcome message to new subscribers can remind them that they requested your emails, educate them about your brand, and reinforce the benefits of your email program. For more information about welcome messages, read How America’s Top Retailers Set the Tone with Welcome Emails.


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



A user defined header element that is injected into the header portion of an email message.

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