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Email – Glossary


A/B Split: A testing tactic by which two variations of the same email are sent against each other, to measure which one generates higher performance. Results can measure against opens, clicks, or other KPIs. Also referred to as split testing.

Above the Fold: The part of an email message that is visible without scrolling. Material in this area is considered more valuable because the reader sees it first. This is originally a printing term used for the top half of a newspaper above the fold. Unlike a newspaper, email fold locations aren’t predictable. Your fold may be affected by the users’ preview pane, monitor-size, monitor resolution, and any headers placed by email programs such as Hotmail, etc.

Acceptable Spam Report Rate: The rate at which you can be reported as SPAM without harming your sender reputation. Anything over 0.1% (1 report per 1000 emails) will get a warning.

Alias: An email alias is simply a forwarding email address that can contain multiple individual recipients. Example would be, which includes all active 250ok employees.

Asynchronous Bounce: An email bounce occurring after the Simple Mail Transfer Protocol (SMTP) has ended and is returned to the sender. Opposite of synchronous bounce. Also referred to as “async bounce.”

Authentication: Authentication is the process of attempting to verify the digital identity of the sender of a communication. In email marketing, there are four main types of authentication: Domain Keys, DKIM, Sender ID, and SPF.


Bacn: Email that has been subscribed to and is therefore not unsolicited (like email spam is), but is often not read by the recipient for a long period of time, if at all. Bacn has been described as “email you want but not right now.” Bacn differs from spam in that the recipient has signed up to receive it. The name bacn is meant to convey the idea that such email is “better than spam, but not as good as a personal email”.

Blacklist: 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 – many times as a free service, sometimes for a fee. There are hundreds of well-known public blacklists. Many ISPs and mailbox providers use blacklists to reject or bulk inbound email, either at the server level or before it reaches the recipient’s inbox. Also referred to as blocklist.

Blast: A term synonymous with sending a large amount of commercial email to a large amount of people. Often implies the same email is sent to a large population, with little to no personalization. Can be viewed as a spammy tactic. Also referred to as “batch-and-blast.”

Block: A refusal by an ISP or mail server not to accept an email message for delivery. Many ISPs 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: A term referring to when an email is not successfully delivered to your recipients ISP or mailbox provider. Emails can bounce for many reasons, and are often categorized as soft bounces, hard bounces, block bounces, technical bounces, or unknown.

Bounce Message: A string of information sent back to an email sender reporting the message could not be delivered and why. Oftentimes bounce messages are vague and difficult to interpret. Example: “550 5.1.1 The email account that you tried to reach does not exist.” Also referred to as error codes, bounce codes, SMTP errors, or SMTP bounce message.

Bounce Rate: Metric used to measure the amount of mail sent versus the amount of mail bounced and was not successfully delivered. Metric can be bounce-type specific (ex: Hard Bounce Rate) or aggregate against all bounces versus mail sent. This is an inexact number because some systems do not report back to the sender clearly or accurately.

Bulk Mail: Refers to mail that is automatically filtered to the bulk folder of a webmail or desktop email client. Also referred to as “Junk” or “Spam” folders in some email clients.


Call to Action (CTA): The link or content in an email message serving as the primary recommendation or action suggested to take. Often located above the fold.

CAN-SPAM Act: Popular name for the U.S. law regulating commercial email (Full name: Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003).

CASL: Canadian anti-spam legislation (CASL) is enacted regulations requiring any individuals or organizations that send commercial electronic messages (CEM) to obtain express consent from all Canadian recipients. … The American Bar Association (ABA) has called CASL “the toughest anti-spam law in the world.”

Certification: A pay-for-play list of senders IPs and/or domains receiving preferential treatment of accepting and inboxing email at various different ISPs taking part in the whitelist. Requires meeting and maintaining certain requirements and standards in order to take part. Most well known whitelist is owned by Return Path, while others include the CSA whitelist (Europe) and SuretyMail.

Click-Through Rate: The ratio of unique click-throughs on a link or links within an email to the total number of recipients of the email, typically expressed as a percentage. The click-through rate does not take into account people who later came to a website in response to an email marketing campaign, so it can be used to measure the direct response only.

Click-to-Open Rate: The ratio of unique click-throughs on a link or links within an email to the total number of unique opens of the email, typically expressed as a percentage.

Cloudmark: 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 ISP’s contribute through their Cloudmark Authority product.

Complaints: An email-centric metric referring to an individual subscriber marking an email as “spam” in their mail client. By marking an email as spam, it generates a complaint back to the sender indicating the individual considers the mail spam. Complaints are the number of spam button hits generated by all recipients of your email. Complaints are negative and hurt sender reputation.

Complaint Rate: The ratio of unique complaints from an email campaign to the total number of delivered emails, typically expressed as a percentage. Recipients of an email can complain using the “This is Junk/Spam” button in their email software platform.

Confirmation Email: An email acknowledgement of a subscription or information request by a company or organization. Often used to verify an email address used at sign-up or as part of a welcome journey. Typically a trigger-based email sent immediately upon sign-up or request for info.

Confirmed Opt-In: A term referring to a company or organization with verifiably confirmed permission for the email address to be included in their specific mailing list. Facilitated by sending an immediate confirmation email asking for the new email address recipient to “confirm” they wish to join the desired mailing list by clicking a link. This is a best practice to avoid bad email address collection and maintain high list quality. Also referred to as “double opt-in” or “COI.”

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”.

Conversion: When an email recipient performs a desired action based on a email message call-to-action you have created. Examples include a subscriber downloading a whitepaper, making a purchase, or signing up for a newsletter.

Conversion rate: A success metric measuring the rate of subscribers who convert to a specific call-to-action or goal of an email campaign.

Customer Relationship Management (CRM): Technology software that helps a company manage customer relationships in an efficient and organized manner.


Dedicated IP Address: An IP address used only by a singular company or sender. Dedicated IPs are not shared with other senders or organizations. Preferred method of senders who want full management of their IP sender reputation.

De-Duplication: The process of removing identical entries or data points from a list or data extension. Most commonly used to remove two or more of the same email addresses on a file or database. Common feature built into many CRMs or data management systems to avoid duplicate entries. Also referred to as “de-duping.”

Deliverability: Refers to the whole subject area of getting your emails delivered to the right place.

Deliverability Rate: Often refers to a sender’s folder placement performance metrics of mail once it is accepted and delivered.  Also referred to as inbox placement rate.

Delivered: Refers to the number of emails that were successful in reaching the subscribers’ inboxes or junk folders. Typically thought of as the Total Number of Emails Sent minus Bounced Emails.

DKIM: DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message while it is in transit. The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for delivery. Technically, DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication.

DMARC: Domain-based Message Authentication, Reporting and Conformance (DMARC) is an email-validation system designed to detect and prevent email spoofing. … DMARC is built on top of two existing mechanisms, Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM).

Domain: A particular organization’s registered name on the Internet. For example,

Domain Name System (DNS): How computer networks locate Internet domain names and translate them into IP addresses. The domain name is the actual name for an IP address or range of IP addresses. Example: Turning into

Domain Keys: Domain Keys in an email authentication system designed to verify the DNS domain of an email sender and the message integrity.

Dynamic Content: Email content changing from one recipient to the next, according to a set of data extensions or variables a marketer has on its audience base. By using dynamic content, you can ensure a more tailored message is reaching your targeted audience. Dynamic content can reflect past purchases, personas, attributes, suggested items, or other content.


Email Change of Address (ECOA): A service that tracks email address changes and updates.

Email Append: A marketing practice taking known customer data (first name, last name, and postal address) and matching it against a vendor’s database to obtain email addresses. Considered a questionable  email collection practice without explicit opt-in permission. Also referred to as “e-append.”

Email Campaign: An email or series of lead nurturing emails designed to accomplish an overall marketing goal.

Email Client: A program used to read and send email messages. As opposed to the email server, which transports mail, an email client is what the user interacts with. Email clients can be software application like Outlook Express and Lotus Notes or webmail services like the ones provided by Yahoo, Hotmail and Gmail. Also referred to in Internet Email protocols as an MUA or Mail User Agent.

Email Filter: Software or algorithms used to block an inbound email based on the sender, content, reputation, or other variables from being delivered. Filters may be applied at the recipient level, at the email client, the ISP, or a combination.

Email Service Provider (ESP): Another name for an email broadcast service provider, a company that sends bulk (volume) email on behalf of their clients.

Engagement: A subscriber-based measurement of how engaged subscribers are with your email program. Engagement is a broad term encompassing how likely subscribers are to open your email, time spent reading emails, clicking, replying, foldering, and other positive explicit actions. Engagement is king with many filtering decisions, as it is a very strong signal from actual subscribers about whether or not the mail is legitimate and should be placed in the inbox.

Explicit Consent: An action taken by an email subscriber to manually opt-in and provide permission to a join a marketer’s email program. An example of explicit consent is requiring a subscriber to click a checkbox to consent to receive an email when completing a sign up form. This is considered the safest option to collect permission and an opt-in from a new email subscriber.


False Positive: A legitimate message mistakenly rejected or filtered as spam, either by an ISP or a recipient’s anti-spam program.

Feedback Loop: A mechanism, process and signal that are looped back to control a system within itself. A feedback loop is sometimes offered by ISPs to companies that wish to remove users that complain about email they receive via the “This is Junk/Spam” button.

Footer: An area at the bottom of an email message that often contains boilerplate information and is consistent across each email. An email’s footer usually contains contact information, company’s postal address, preference center, unsubscribe links, and privacy policy.

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


General Data Protection Regulation (GDPR): Instituted on May 25, 2018, it replaces the EU Data Protection Directive adopted in 1995. The primary purpose of the GDPR is to protect the personal data of residents of countries within the European Union (EU).

Google Postmaster Tools (GPT): Google’s web-based set of free tools designed to provide senders and deliverability professionals insights into various reputation and data metrics for a sender, based on domain. GPT provides domain/IP reputation, spam rates, authentication info, encryption, and more based off a sender’s domain.

Gravestoning: An action taken by major ISPs like AOL, Gmail and Comcast where an inactive email is changed into a spam trap.

Graymail: A term used to describe opt-in, permission-based email an individual does not routinely engage with and may not want. It differs from spam in that the recipient consented to it, and its name derives from being between spam and non-spam.

Greylisting: A technique used by some ISPs 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.

Header: Lines identifying particular routing information of an email message, including the sender, recipient, date subject, authentication, and SMTP relays, to name a few.  Headers are found in a delivered email by viewing source or viewing original in the mail client.

Honeypot Spam Trap: A nickname for a spam trap email address that is created by an ISP or mailbox provider for the sole purpose of catching spammers. These traps never sign up for mail and are spread out on the internet, waiting to be scraped or harvested by spammers. These are the most severe types of spam traps. Also referred to as “pristine” spam traps.

HTML Message: Email message which contains Hyper Text Markup Language syntax and encoding. HTML Messages must be properly encoded and receiving email clients (MUAs) 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.


Implicit Consent: When an individual provides his or her email address during a business transaction or communication, but does not necessarily opt-in specifically for email. An example is a pre-selected checkbox, or requiring an email address as a required field to download an asset but not implying what it will be used for. Implied consent is not considered best practice.

Inactives: Also referred to as “non-responders”. Defined as the portion of your email recipient list 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 junk folder. Calculated as Number of Emails Delivered to the Inbox divided by Total Number of Emails Sent.

Infrastructure: 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 Mailing Transport Agent (MTA).

Internet Service Provider (ISP): A company in the business of providing access to consumers and business to the internet.

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.

Invalid Email: An email address flagged as invalid by the Omeda system is one that has reached its threshold of bounces – either hard or soft. The bounce counter threshold is a custom setting on a database level. If the email does get delivered, the bounce counter is reset to zero and the invalid flag is removed.


Junk Mail Reporting (JMR): This is the name of Microsoft’s Feedback Loop program.


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. Some list owners also use an Email Change of Address service to update old or abandoned email addresses (hopefully with a permission step baked in) as part of this process.

List Rental: The process in 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.

List Purchase: The process in 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-Unsubscribe: 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, Windows Live and Cloudmark.


Multipurpose Internet Mail Extension (MIME): An extension of the original Internet email standard allowing users to exchange text, audio, or visual files.

MTA (Mail Transfer Agent): 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.

MUA (Mail User Agent): A Mail User Agent is a client application that allows users to send and retrieve email from their computers. Common MUAs include Microsoft Outlook, Eudora and Netscape Messenger. MUA’s are the component within the SMTP system that is responsible for creating email messages for transfer to an MTA. Also referred to as an “email client”.

Multi-Part MIME: Bundles together a simplified plain text version of your email along with the HTML version. Most mail clients supporting this format will display the HTML version, but mail clients that can’t will show the text version instead.

MX Record (Mail Exchange Record): An MX Record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed using the Simple Mail Transfer Protocol (SMTP).


Open Rate: The number of HTML message recipients who opened your email, usually as a percentage of the total number of emails sent. The open rate is considered a key metric for judging an email campaign’s success, but it has several problems. The rate indicates only the number of emails opened from the total amount sent, not just those that were actually delivered. Opens also can’t be calculated on text emails, as it is dependent on image downloads. Also, some email clients allow users to scan message content without actually opening the message, which is falsely calculated as an open.

Opt-in: 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).

Opt-out: 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.


Phishing: 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 data.

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.

Postmaster: 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.

Preference Center: A landing page providing an email subscriber a centralized and easy way to manage their email preferences, including cadence, content, and volume.

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 (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. Typically thought of as more accurate than open rate, since read rate is not dependant on image downloads.

Receiver: A generic term used to describe an Internet Service Provider or network that accepts and delivers large amounts of email.

Recycled Spam Traps: Email addresses that were once used by a real person. These email addresses are abandoned email accounts that are recycled by ISPs as spam traps. Before turning an abandoned email address into a spam trap, ISPs will return unknown user error codes for a year. Once ISPs 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 your data hygiene process is not working.

Re-engagement Campaign: An email campaign sent to inactives, or non-responders, in an attempt to win them back and get them engaging with your emails again in the form of opens, clicks, and conversions. A re-engagement campaign can be sent to inactives as a stand-alone campaign, or as a series of campaigns.

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 can be an automated or unmonitored email address used only to send messages to a distribution list. “Reply-to” should always be a monitored address.

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. ISPs 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 ranking tool.

Responsive Design: Using a CSS3 coding technique called media queries, Responsive Design allows your email to automatically re-format and re-size itself 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 call-to-action of the email is easily found, and can change various other elements of the email, including text size and color, background images and background color.

Return-Path: 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, and it’s the address to which any undeliverable message notices (bounces) are sent.

Reverse DNS (rDNS): The process in 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.


Seeds: Seeds are a collection of email addresses used to test rendering, delivery, and inbox placement of emails. Seeds can be personal test email accounts, or select vendors offer a range of seed lists covering domains all over the globe for a scalable way to test delivery and inbox placement.

Segmentation: The email marketing technique of breaking a list up into different segments. You can segment a list dozens of different ways, including by subject preferences, last opened date and more.

Sender ID: The informal name for an anti-spam program that combines two existing protocols: Sender Policy Framework and CallerID. SenderID authenticates email senders and blocks email forgeries and faked addresses.

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

Server: 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: An IP address used for sending by more than one sender at a time. Less ideal than a sender using their own dedicated IP address, as senders leveraging shared IPs have little control over their IP sender reputation, as its shared with numerous other senders. Some ESPs offer shared IP pools, with multiple clients sending from multiple shared IPs. Often a more cost effective option when selecting an IP.

Simple Mail Transfer Protocol (SMTP): A server-to-server mail transfer protocol. Examples are Sendmail, Postfix, and Qmail.

Smart Network Data Services (SNDS): Offered by Windows Live Hotmail, SNDS provides data to senders based on actual mail sent to Hotmail subscribers. Metrics reported on include complaints, SmartScreen filter results, and spam trap hits.

Soft Bounce: Email sent to an active (live) email address but which 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.

SpamAssassin: A common spam filter using a variety of spam detection techniques, including DNS-based spam detection, content screening, and authentication checks to score emails. SpamAssassin runs these tests to calculate an aggregate score that can be used to determine whether or not a message is spam. The popular choice among those employing SpamAssassin is to mark anything with a score of “5” or more as spam.

Spam: 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.

Spamhaus: One of the more impactful and globally known anti-spam organizations. Based in London, Switzerland, and  Geneva, the non-profit organization tracks email spammers and spam-related activities. The Spamhaus project is responsible for compiling several widely used anti-spam blacklists and blocklists.

Spam Filter: A mechanism used to identify spam email and keep it out of the recipeint’s inbox.

SpamCop: A blacklist and IP address database, formerly privately owned but now part of the email vendor Ironport. Many ISPs check the IP addresses of incoming email against SpamCop’s records to determine whether the address has been blacklisted due to spam complaints.

Spam Trap: A spam trap looks like a real email address, but it doesn’t belong to a real person nor is it used for any kind of communication. Its only purpose is to identify spammers and senders not utilizing proper list hygiene.

Spoofing: The malicious act of falsifying the sender email address to make it appear as if an email message came from somewhere else.

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.

Spam and Open Relay Blocking System (SORBS): Owned and operated by Proofpoint, Inc., providing free access to its DNS-based Block List (DNSBL) to effectively block email from more than 12 million host servers known to disseminate spam, phishing attacks, and other forms of malicious email. SORBS is a common blocklist many senders hit, but is a low impact blocklist.

Spoofing: 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.

Subscribe: 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.

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

Sunset Policy: A term referring to the defined criteria when an email marketer considers an email address no longer active and will suppress it in the future from commercial emails. Usually, a sunset policy is defined by a length of time when an email address has shown no engagement.

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. Required by CAN-SPAM.


Tabs: Refers to a webmail client’s interface that offers multiple inbox tabs instead of a singular primary inbox. Most notable example is Gmail’s tabbed inbox interface, which offers a Promotions tab, Social tab, Primary tab, and others that can be customized in settings. Also referred to as tabbed interface or inbox tabs.

This is Spam Rate: The rate of emails that are marked as junk/spam by recipients, typically expressed as a percentage over total number of emails delivered.

This is Not Spam Rate: The rate of emails that are recovered from the junk/spam folder by recipients, typically expressed as a percentage over total number of emails delivered.

Throttling: The practice of regulating how many email message a broadcaster sends to one ISP or mail server at a time. Some ISPs bounce email if it receives too many messages from one sending address at a time.

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 U.S. Postal Service address or an unsubscribe link.


UCEPROTECT: An IP-based spam blacklist primarily used in Europe to catch and block spam and abuse. Usually low impact to delivery when listed.

Unsolicited Commercial Email (UCE): Spam or junk mail.

Unknown User: Bounce error code generated by an ISP when an email address is not registered in its system.

Unique Open Rate: The percentage of unique emails opened versus the number of emails delivered. A single subscriber opening an email multiple times would only count as one unique open.

Unsubscribe: The act of requesting to be removed from a marketer’s email program or newsletter. Facilitated by clicking the unsubscribe link or requesting manually to be removed to by the sender. Unsubscribes do not negatively impact sender reputation like spam complaints do.


Warm-up: A pivotal period of establishing a sender’s identity and reputation on new IP(s) and/or domain(s). The term refers to the period of time where senders must prove themselves as legitimate senders when sending off new IPs/domains to prevent ISPs and mailbox providers from bulking or blocking their emails. Requires a slow ramp of volume to highly engaged subscribers to help bolster positive reputation for up to six weeks.

Whitelist: 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.

WHOIS: A publicly available and widely used internet record listing identifying who owns a domain and how to get in contact with them.


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

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