Back to main menu



Lost in all the technical jargon? Find definitions for common email and technical terms and learn more with our related resources.


An Application Programming Interface (API) is a programmatic way for applications to communicate with one another. An email API is a specific type of API that allows email developers to integrate their applications with email service providers (ESPs). This lets email developers send and receive emails and take advantage of ESPs’ email analytics and deliverability services from within their own environments.

Abandoned cart email

An abandoned cart email is a triggered (or transactional) email sent to customers when they leave a web shop without purchasing the items in their shopping cart. It acts as a friendly reminder to encourage potential customers to take action, converting them to buyers after they have navigated away from your site.


An allowlist, formerly known as a whitelist, is a list of approved senders or recipients that are permitted to send emails to a particular inbox. It is used to ensure that emails from known and trusted sources are allowed to pass through a filter.


The “Archives” folder is listed in an email client alongside other standard folders like the inbox and spam folders. The folder is where users can store old messages, reports, and letters. In email marketing, archiving is the process of moving emails from an inbox to a specific Archives folder. It’s important to periodically archive emails as it declutters the inbox.

Authentication protocols

An authentication protocol is a cryptographic standard that validates the sender and receiver. It acts as an ID card to verify a brand’s identity to both email servers and inbox service providers (ISPs), ensuring the brand is credible.

Automated emails

Automated emails are email messages automatically sent by a CRM or software, usually triggered by a user’s actions. These emails are also called transactional emails.

Automation flow

Automation flow is a practice of mechanizing tasks like responding to customer inquiries, creating sales invoices, and testing new products. This allows brands to free up their employees’ time to focus on high-value tasks like developing marketing strategies and communicating with customers.


An autoresponder is a feature that automatically responds to incoming emails. Users generally rely on an autoresponder when they’re out of office or dealing with a large volume of emails. Brands use an autoresponder to send emails confirming certain user actions, like placing an order or receiving a payment.


Brand Indicators for Message Identification (BIMI) is an email authentication that requires DMARC implementation. It allows senders to include a branded sender image in their emails.

Behavioral email

A behavioral email, or a triggered email, is a message sent in response to certain user actions, like abandoning a cart, usually after users interact with a brand’s web application or social media. Behavioral email marketing is a great way to boost user engagement by reminding recipients to take action.

Bot detection

Bot detection is the practice of analyzing the traffic that visits a company’s website,  mobile app, or API to detect malicious bot activity. It protects brands from script injections and list validation attacks by hackers and scammers. Email professionals usually collect emails from new subscribers with a webform. After this, validating the email addresses ensures emails are sent to real users who want to receive a brand’s messages and content.

Bounce rate

Bounce rate measures the percentage of emails that bounce back, or the number of emails that couldn’t be delivered to users over the total number of emails sent. The formula is:


The Controlling the Assault of Non-Solicited Pornography And Marketing (CAN-SPAM) Act is a regulation enacted by the United States in 2003 to limit a company’s ability to send unwanted or spam emails to customers. While it’s never recommended to send spam to recipients, CAN-SPAM makes it illegal.


The California Consumers Privacy Act (CCPA) is a 2018 California regulation that focuses on protecting consumer data rights for California residents. These rights include the right to know about the personal information businesses are collecting, the right to obtain the data collected, and the right to delete the information provided (with some exceptions).

CDP (Customer Data Platform)

A CDP (Customer Data Platform) is a software tool used to manage data collected from customer interactions with web apps or emails.


A chatbot is a software tool that provides instant automated responses to users’ messages. These are embedded in web pages and powered by a software called conversational AI. Depending on how advanced the software is, these bots mimic a human being and triage customers’ needs. After completing the first few customers’ requests, the chatbot either directs the users to the next steps or connects them to a human representative. They’re a staple of reliable user experience and user interface design. Brands can also set up an email chatbot to respond to customers’ emails, automating responses and simplifying interactions.

Click-through rate (CTR)

A click-through rate (CTR) is the proportion at which recipients follow a call-to-action (CTA) and visit the web page (or landing page). It’s a great indicator of whether a CTA works or if subscribers find the content engaging. When expressed in a formula, the CTR factors in the total number of clicks divided by the total number of emails sent and multiplied by 100. Here’s how:

Client relationship management

Client Relationship Management (CRM) is the process of determining when, where, and how to interact with clients to evaluate and manage client interactions. Brands use CRM tools – software that automate customer relationships – to set up specific touchpoints in the customer journey or events that trigger automated emails. For instance, if a new subscriber browses a website but leaves without taking any actions, the CRM registers this as a touchpoint and sends an automated email to follow up.

Conversion rate

Conversion rate is a marketing metric that assesses the number of users who made a purchase after opening an email versus the total number of emails sent out. The conversion rate formula is the number of recipients who followed a call-to-action (CTA) divided by the number of emails delivered expressed in percentage:


DKIM, which stands for Domain Keys Identified Mail, is an email authentication protocol that allows a mail server to check that an email message was sent and authorized by the sending domain. It acts as an identity card for email sending to reassure mail servers that the sender is authentic and credible.


Domain-based Message Authentication, Reporting, and Conformance (DMARC) is an authentication method designed to stop bad actors from impersonating you. DMARC acts as an “identity card” to mail servers and inbox service providers (ISPs), allowing both to validate the sender’s identity to prevent cybercriminals from spoofing emails.

DMARC reporting

Domain-based Message Authentication, Reporting, and Conformance (DMARC) reporting is the process of sending information and metrics about emails sent from a domain – whether the emails are authenticated by SPF and DKIM or not. These reports also inform brands if any spammers are using their domain names to send malicious emails to recipients.

DNS TXT record

A Domain Name Service (DNS) TXT record is an authentication protocol used to verify domain ownership to other sources outside your domain, like inbox service providers (ISPs), mail exchange servers, and email clients. A DNS record is a *.txt file that verifies domain ownership.

Distribution channel

A distribution channel is a network of wholesalers, retailers, and buyers that brands rely on to manufacture products and deliver and market them in end markets. Here are the three types of distribution channels:


Domains are destinations on the internet that are controlled by individuals or organizations. Searchers look for domain names by entering the address or a keyword in a browser’s search bar. For instance, Mailgun’s domain name is Companies send emails from their domains. A domain’s reputation strongly impacts whether or not a brand’s email messages will be placed in its recipient’s inbox, which is also known as inboxing rate.

Double opt-in

Double opt-in is the process of adding new subscribers to an email list by reconfirming a recipient’s consent to receive emails. Here’s a step-by-step process:

Email analytics

Email analytics are a way to track statistics like open rate, conversion rate, and click-through rate, all of which allow brands to determine if their current email strategy is working or if they should pivot to something else. These metrics are also an important part of an email program, as brands gauge their email campaigns’ health and ROI to assess market strategies and plan for growth.

Email automation

Email automation involves automating messages for an email program so they are received by the right people at the right time. These triggers include messages like:

Email blast service

An email blast service enables brands to integrate email with their web apps and send email campaigns at scale, and provides metrics on campaign performance to help you pinpoint effective strategies for sending.

Email bounce

Email bounces occur due to temporary or permanent email delivery failures. Temporary delivery failures, also called soft bounces, often occur due to server downtime, interrupted internet connections, and temporary failures. Permanent delivery failures, or hard bounces, occur due to incorrect email addresses, email servers that reject emails, and invalid domain names.

Email compliance

Email compliance is the process of ensuring data practices conform to data protection regulations, like the EU’s General Data Protection Regulation (GDPR), the Health Insurance Portability and Accountability Act (HIPAA), and the California Consumers Protection Act (CCPA). Becoming compliant involves these regulations for obtaining, storing, and processing customer data. Also, being compliant with a particular legislation or security standard signals that a company protects its customers’ personal data rights, and builds reputation with inbox service providers (ISPs) improving email deliverability.

Email deployment

Email deployment is the process of sending emails to a predetermined email list. It’s typically associated with email marketing practices where businesses send high volumes of emails at once. But email blasts aren’t necessarily good for a brand’s IP reputation and may negatively affect deliverability, as inbox service providers (ISPs) may perceive it as spam. To combat this issue, brands leverage email deployment services like Mailgun to optimize send time and manage the volume of emails sent to ensure emails land in subscribers’ inboxes. These email deployment platforms rely on customer data to ensure they deliver emails at an optimal time.

Email encryption

Email encryption is the process of using secure cryptographic protocols to ensure email messages are encrypted before, during, and after sending. It’s important for email security because it prevents hackers from employing man-in-the-middle attacks, which put sensitive personal details of both brands and customers at risk.

Email envelope

An email envelope is the “envelope from” address of an email message, and it’s used for routing. Every email contains two “from” addresses, “envelope from” and “header from.” The “header from” address is what people normally think of as the “from” address. It’s visible to all email users and displays in the “from” field of an email. This address is read by users and email clients but isn’t used for email delivery. The “envelope from” is a return path, which is the return address hidden in the email message header that instructs mail servers or inbox service providers (ISPs) where to return messages if they bounce. This address is used for email delivery.

Email gateway

An email gateway server processes all incoming and outgoing emails to keep brands’ internal servers safe. It’s a key part of an email infrastructure that protects internal and external communication. Secure email gateways monitor sent and received emails to prevent phishing emails, which spoof legitimate emails to reach a brand’s internal servers. They protect brands against malware and fraudulent content.

Email harvesting

Email harvesting is the process of maliciously obtaining a large number of email addresses for sending spam or malware. Specialized email harvesting tools like harvesting bots crawl through websites and exploit vulnerabilities to obtain lists of email addresses. At Mailgun, we recommend complying with all data collection and storage regulations and organically building an email list to avoid spam traps.

Email integrations

Email integration is the process of unifying various systems, tools, and software to create a seamless workflow for email marketing. Companies use email integration in websites, web or mobile apps, and more. They bring together email development tools like drag-and-drop editors, email service providers (ESPs) responsible for delivering email, and analytics software that track an email program’s health.

Email logs

Email logs contain email data on each message that has been sent, including the sender’s details, recipient’s email address, timestamps, and any error codes. Logs can also link to reports that track marketing metrics like delivery, bounce, and open rates. These gauge an email program’s health, helping senders analyze why their emails have failed to reach recipients’ inboxes, sending trends, and performance issues impacting sender reputation. Email logs let senders perform deliverability audits to see where the issues might be before they impact deliverability.

Email open rate

Email open rate refers to the percentage of emails brands send that recipients open. When expressed in a formula, it’s the number of emails opened divided by the number of emails sent and multiplied by 100.

Email order confirmation

An email order confirmation is sent to a customer to confirm an order they’ve placed via a web store. It’s a transactional email, or an automated response based on users’ actions. These emails usually consist of the following details:

Email parsing

Email parsing is the process of extracting data from incoming emails, carried out by a software using an Application Programming Interface (API). The extracted data could be recipient contact details, order numbers, and leads from an email’s HTML header and body code or attachments. Brands use email parsing to collect and send email data to web applications for more efficient workflows.

Email routing

Email routing is the process of designating an email to be a routing address to handle incoming email traffic. Email senders don’t need to know the routing address to send messages to an email account, as the address is only seen and used by the receiving mail server to point messages to a certain destination. In other words, email routing involves forwarding all incoming emails to the routing address.

Email server

An email server is a system responsible for sending, receiving, and storing messages. The server is accessed by users through a client program, such as Microsoft Outlook or Apple Mail, using either the internet or a secure local network. The email server uses protocols, such as SMTP and IMAP, to communicate with other mail servers and deliver messages to recipients. Depending on the email server, there may also be built in filters for spam and virus detection.

Email service providers

An email service provider (ESP) is a software provider that facilitates integration between email capabilities and web or mobile applications. It allows organizations to integrate email sending with their web apps via an Application Programming Interface (API) or Simple Mail Transfer Protocol (SMTP). Brands can then send emails at scale while managing their IP reputation and email-sending schedules.

Email spoofing

Email spoofing is a method used to trick recipients into opening spam emails as part of a phishing attack. Cybercriminals forge email headers to mimic trusted sender identities and trick an email client into displaying a fake “from” email address. When recipients see the fake email address, they might presume the email is from a person or brand they know and trust. Once they open the email, the hacker tries to convince the victim to share sensitive personal details, resulting in cybercrime.

Email subscriber

An email subscriber is a user who has opted to receive email messages from a specific sender. A key part of building up an email program is growing and maintaining a healthy  list of email subscribers. Mailgun recommends implementing double opt-ins, which ask recipients to twice confirm they want to be added to a mailing list. This helps senders grow their email lists organically and maintain high engagement from users not likely to mark them as spam.

Email validation

Email validation, or email verification, is the process of checking that the email addresses on an email list are valid and not spam traps to maintain email list hygiene. Brands perform email validation when new subscribers sign up to be on their email lists. Companies periodically clean email lists through email validation, identifying invalid email addresses, possible bad actors like spam traps, or low-quality emails like info@ or admin@ types of email addresses to keep deliverability high.

Email validation API

An email validation API is a specific API offered by some ESPs like Mailgun, that automatically checks your email list for errors, common typos, domains, etc. to validate your email addresses and ensure your email list is clean. This contributes to a healthy sender reputation.

Email worm

An email worm, or mass-mailer, is a type of an internet worm (or virus) cybercriminals use to infiltrate their victims’ inboxes. Hackers usually deploy worms as attachments to fraudulent email messages. These attachments allow the worm to distribute itself in infectious executable files. Email worms self-download files and delete all security software on users’ devices. They also pose a grave security threat to email programs and brands’ IT infrastructure by creating their own copies and attaching to emails.

End user

An end user is an individual who uses a product or service. The term “end user” is generally interchangeable with “customer,” but customers can be retailers buying from wholesalers or third parties. End users are the customers who ultimately use the product.


The General Data Protection Regulation (GDPR) is a 2018 European Union data privacy protection act that provides guidelines to organizations managing customer data on how to collect, store, and use it. These organizations include businesses that operate in the EU or interact with the data of persons from the EU. By regulating the processing of private information, GDPR seeks to provide consumers with more transparency of what data is collected and how it’s used. Organizations not complying with the GDPR are liable to pay hefty penalties and fines.


Email greylisting is a system used to filter incoming emails by temporarily rejecting them if they are from an unknown sender. The sender is then put on a temporary list and must resend the message after a certain period of time.


Greylisting is a method of email filtering used to reduce spam that leverages greylists. It works by temporarily rejecting messages from unknown or suspicious sources and allowing delivery if the sender is legitimate.


The Health Insurance Portability and Accountability Act (HIPAA), enacted by the United States Congress, is a 1996 regulation that protects patients’ health information and medical histories. The Act restricts health institutions from passing on this information to anyone without the patient’s consent and knowledge. It lays out three main rules concerning data privacy, data security, and data breaches. HIPAA is the forerunner of data protection regulations like the more recent General Data Protection Regulation (GDPR) by the EU and the California Consumers Privacy Act (CCPA).

Hard bounce

There are two types of email bounces: hard bounces and soft bounces. While soft bounces are temporary delivery outages that often resolve over time, hard bounces are permanent delivery failures to recipients’ inboxes.

Header / Email header

An email header is a snippet of code that always precedes the email bodyt. It contains information about an email and its journey from the sender to the recipient and valuable feedback about the sender’s deliverability, including the following:


Internet Messaging Access Protocol (IMAP) is the internet standard responsible for retrieving emails from a receiving email server and placing them in recipients’ inboxes.


IDLE is an Internet Messaging Access Protocol (IMAP) extension. It allows an IMAP client to tell the IMAP server that it’s ready to accept real-time notifications and new messages. IMAP IDLE enables an email client to tell a receiving mail server it’s ready to receive an email, meaning that subscribers don’t have to hit a refresh button to “pull” new emails into their inboxes. Instead, they’ll automatically see new messages when delivered.

IP address

An Internet Protocol (IP) address is a string of numbers that identifies any device connected to a network. An IP address can be used to identify where data originates as well as its destination.  These addresses facilitate communication between networked devices.

IP warmup

Internet Protocol (IP) warmup is the process of gradually ramping up the volume of emails sent from a dedicated IP address to build a domain’s reputation among inbox service providers (ISPs). Typically, when a brand suddenly starts sending multiple emails, ISPs suspect its IP address to be “spammy” and question if the brand is truly sending high-quality content to recipients, which is why brands must warm up their IPs.

Inbound processing

Inbound processing is a feature that enables users to send inbound messages to an email service provider (ESP) like Mailgun. In our case, the Transactional API handles the processing and parsing of the inbound message and delivers the results to your web app via a webhook. This is useful because you can allow users to directly reply to your outbound messages, reducing user friction and increasing the likelihood of scoring a conversion.

Java language

The Java language, not to be confused with JavaScript, is a scripting language used to send emails via Simple Mail Transfer Protocol (SMTP) or an email Application Programming Interface (API). While web developers rely on JavaScript in web development, we don’t recommend using JavaScript in email development.


List-unsubscribe is an optional process included in the email header, enabling subscribers to opt out of an email list without clicking on an unsubscribe link. At Mailgun, we always recommend providing subscribers an option to stop receiving emails, so they don’t mark emails as spam. If they do, it can tank a domain’s reputation and email deliverability.

Looping mail

Looping mail is the unintentional process of autoresponders sending automatic replies that end up in a loop. For example, if a company sends an email to its customer but the customer has an  autoresponder set up, it will send an automated reply to the brand. But, if the brand also has an auto-reply on, the automated replies will continue responding to each other. This infinite exchange of automatic responses creates an email loop. While email automation has many benefits, an automated response loop can occur if brands set up automation incorrectly.

MIME (Multipurpose Internet Mail Extensions)

Multipurpose Internet Mail Extension (MIME) increases the sending capabilities for non-ASCII (non-American Standard Code for Information Interchange) data files like audio, video, images, and apps within emails. MIME is an extension of the Simple Mail Transfer Protocol (SMTP).

MX record

A Mail Exchange (MX) record describes how an email should be relayed and routed to the end user. It’s also called a Domain Name System MX record, or DNS MX record, because it directs email from a domain to a server via the Simple Mail Transfer Protocol (SMTP). This ensures the email is relayed from the sender to the receiver.

Mail Transfer Agent (MTA)

A Mail Transfer Agent (MTA) is an email server software that receives and relays emails. An MTA receives messages from another MTA or from a Mail User Agent (MUA), which is a computer application that end users use to access or send emails. Once an MUA relays an email to an MTA, the MTA takes over and sends it to the receiving MTA via Simple Mail Transfer Protocol (SMTP). If the sender seems spammy, inbox service providers (ISPs) will restrict the email delivery, potentially impacting the brand’s deliverability.

Mail user agent (MUA)

A mail user agent (MUA), also called an email client, is a mail client application that grants users access to a mail server in order to create, send, and receive email messages. MUA examples include Microsoft Outlook, Gmail, and Apple Mail.

Marketing automation

Marketing automation refers to the process of automating steps in a brand’s communication strategies. In email programs, marketing automation boosts efficiency and improves ROI by integrating with Customer Relationship Management (CRM) software, mechanizing lead nurturing, and allowing senders to trigger automations based on recipients’ engagement with the emails.

Message ID

A message ID is the unique identifier issued by an email service provider (ESP) used to track a specific message. This ID is composed of a string of random alphanumeric characters, and it’s unique to the message sent to recipients, regardless of how many recipients there are. It also includes the email client’s name to indicate the sender. Here’s an example:


Omnichannel is a sales and marketing approach that integrates a user’s shopping experience across multiple marketing channels, including web apps, email, SMS, and social media. In an omnichannel strategy, a user visits a company’s website and signs up for its monthly newsletter. With their permission, the company follows up with a coupon or more product details. At the same time, the company deploys social media ads to catch their attention. If this drives the customer back to the website, it’s an opportunity for the company to analyze the customer’s behavior. These different marketing channels are employed to guide a customer down the path to purchase.

PHP language

PHP language, initially called Personal Homepage but now a recursive acronym for PHP: Hypertext Preprocessor, is a scripting language embedded in HyperText Markup Language (HTML) for web pages. It’s used to develop web servers.

POP server

Post Office Protocol (POP) is a standard protocol, like Internet Message Access Protocol (IMAP), responsible for retrieving emails from an email server to deliver them to recipients’ inboxes. A POP server is a special type of email server responsible for syncing Gmail messages with compatible email clients, like Apple Mail, Microsoft Outlook, or Thunderbird. It allows recipients to use single sign-on for all email accounts, providing them access to all their email accounts from one email client.


Also known as an allowlist (formerly whitelist), passlists are lists of email addresses, IPs, and domains that are allowed to bypass certain security measures, such as spam filters.

Personal Folders File (. PST)

A Personal Folders File (. PST) is a Microsoft Outlook file that stores data from emails, calendar events, and other items from a Microsoft Exchange mailbox. Storing the contents of an Exchange mailbox creates free storage space for incoming emails. This allows recipients to search emails and gives them the flexibility to design their own . PST system as per their needs, allowing them to organize their space the way they prefer it like combining inboxes or splitting folders.

Personalized email

A personalized email is a message tailored to each individual user. Brands draw on customer data like their behavioral aspects, geographic location, or demographics to create personalized emails catering to customers’ needs. Customers are more likely to be interested in emails that speak to their requirements, boosting the brand’s user engagement metrics.


A postmaster is a mail server’s administrator and has the associated email address Mail servers route messages about errors in email sending or processing to this email address. Every domain that supports the Simple Mail Transfer Protocol (SMTP) for sending emails must have a postmaster address setup.

Public key cryptography

Public key cryptography, also known as public key encryption or asymmetric cryptography, is the process of using a pair of keys to encrypt data. While one key is a public key, the other is a private key that’s stored internally. These keys are used to authenticate a sender’s identity or encrypt data.


Python is a general-purpose scripting language that’s often used in web application development, desktop application, and data analysis. Developers also use Python to integrate with APIs from email service providers (ESPs). For instance, you can write a Python script to integrate Mailgun’s Email API with your web application. You can also script your email sends and take advantage of Mailgun’s powerful email deliverability features, email list management tools, and send-time optimization functionalities.

Quarantined email

Quarantined emails are emails flagged as they make it to subscribers’ inboxes. Emails go into a “quarantine state” if a message looks partly spammy or potentially malicious, and the inbox service provider (ISP) in charge of placing the message in the reader’s inbox isn’t really sure. Here, subscribers have the power to move quarantined emails into their inbox or permanently delete it.

Queued email

Senders can voluntarily choose to queue emails. This enables them to take advantage of asynchronous sending and optimize send time based on customer behavior. It also manages sending over an IP. If a sender sends a high volume of emails without warming up its IP, this might flag the message as spam content to ISPs.


REpresentational State Transfer (REST) is an architectural-style Application Programming Interface (API) that centers around manipulating resources. RESTful APIs boil down to a simple request-and-response pattern. Brands interact with a RESTful API by using a program that uses the API to request data from the server. Then, the API returns data from the server, and the client displays the results to the user. These requests are generally made using the Hyper-Text Transfer Protocol (HTTP).


ROI, or Return on Investment, is a metric used to determine an investment’s efficacy, calculating whether it has earned a profit or incurred a loss. When expressed in a formula, it accounts for the profit or loss divided by the total investment and multiplied by 100. It’s expressed as a percentage. Here’s how:

Rate limiting

Rate limiting is a technique to restrict traffic on a server. In email terms, rate limiting in Application Program Interfaces (APIs) refers to how an email API might limit the number of calls an organization can make during a set period, impacting the number of emails they can send during that time frame. Rate limiting is a way to ensure fair use and is also an excellent way to protect infrastructure from hackers who might spam an open API endpoint on a brand’s website with too many requests. Organizations typically rate limit web app APIs to secure their websites from cyber attacks and manage overall network traffic.

Reactivation email

A reactivation email, or re-engagement email, is sent to users no longer engaging with a brand (usually after a period of about three months). Even if users initially sign up to receive the brand’s communications, they may change their minds over time and stop opening marketing emails. This is when the brand can entice them back with promotional coupons, discounts, and offers.


A redirect is a method to divert recipients to a web page different from the one they initially intended to visit. This could happen due to a broken original web page or a change in the web page’s URL.

Return path

A return path is used to specify where bounced emails are sent and is placed in the email header. It’s an SMTP address separate from the sending address. This is a good practice for email delivery, as it gives bounced emails a place to land – other than in an inbox – making it easier to avoid sending notifications to bounced addresses. However, it’s important for authentication to ensure that the return path domain is the same as the sending domain.

Ruby programming language

The Ruby programming language is an open-source, object-oriented scripting language that leverages reusable code to speed up development. Its applications include:

SMS message

Short Message Service (SMS) facilitates sending text messages of up to 160 characters. In email marketing, SMS messages are sent to interested users on their mobile devices. These messages include notifications like follow-up texts, promotional coupons, or order confirmations.

SMS messaging

SMS, which stands for Short Message Service, involves sending targeted, triggered text messages to audiences offering promotions, incentives, or reminders to complete their purchase. SMS messaging usually relies on text, with the occasional use of images or emoji.

SMTP authentication

SMTP authentication, also known as SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) that adds a layer of security to email sending. SMTP is used alongside email APIs to send emails. SMTP authentication works by requiring the email sender to log in using a supported authentication method. Then, the mail server sends the email message after the sender has successfully logged in.

SMTP relay

Simple Mail Transfer Protocol (SMTP) relay, also called email relay, is the process of relaying messages from one server to the other using the SMPT protocol. SMTP relay helps to ensure that email messages are delivered quickly and reliably, even if the sender and recipient are using different email systems.

SMTP server

A Simple Mail Transfer Protocol (SMTP) server is an outgoing mail server that relays email messages brands send using the SMTP protocol. Typically, brands use two ways to send emails: SMTP or email Application Programming Interface (API). When emails are sent via SMTP, the email client relays the email via an SMTP port to an SMTP server, which authenticates the message before sending it to a receiving mail server. Lastly, the recipient’s email client retrieves the message using the Internet Message Access Protocol (IMAP).

SPF (Sender Policy Framework)

SPF, or Sender Policy Framework, is an authentication protocol that validates a sender’s identity to inbox service providers (ISPs). SPF records detail all the authorized mail servers that can send emails from a particular domain. SPF authentication provides rules to ISPs so they can recognize whether or not an incoming email message is being sent from an authorized sender. ISPs use these rules to accept, reject, or flag all incoming messages.

SSL Certificate

A Secure Sockets Layer (SSL) certificate is an encryption standard that ensures communication between two parties stays safe and secure. Websites secure with SSL certification contain an “s” after “HTTP,” like Like web pages, brands can secure email communication through SSL certification or other encryption standards like Transport Layer Security (TLS) or end-to-end encryption.

Seed test

A seed test is the process of sending emails to a select group of email addresses to troubleshoot potential email problems and gauge inbox placement rates before actually rolling out an email campaign. These tests provide brands with insights into how many emails reach  the inbox, spam folder, or no folder at all. Brands can then catch problems before they happen and determine if authentication protocols and sending infrastructure are set up to handle the email-sending traffic.

Send Time Optimization

Send Time Optimization (STO) is a feature that allows brands to send emails at the optimal time to reach every recipient. This boosts engagement rates as email messages are sent to recipients at times they’re most likely to interact with the email.

Sender ID

A sender ID is an alphanumeric code unique to each email sender. It acts as an authentication record to validate brand identities to inbox service providers (ISPs) and mail servers to prevent emails from being misinterpreted as spam.

Single opt-in

Single opt-in is the process of allowing users to subscribe to email programs in one step. It typically appears as a popup box as soon as a user visits a website, asking the user to subscribe to the brand’s communication only by sharing their email address.

Smart Network Data Services (SNDS)

Smart Network Data Services (SNDS), by Microsoft Outlook, is a free tool that provides insights into how Microsoft Outlook views the health of a brand’s IP address. SNDS also provides information about a brand’s bounce percentage, spam trap hits, and the number of emails sent from the brand’s IP address – all of which help the brand assess its sender reputation and email deliverability. Good sender reputation enables inbox service providers (ISPs) to validate the brand’s domain name and place emails in recipients’ inboxes without speculation.

Soft bounce

A soft bounce is a temporary email delivery failure that occurs for various reasons like a server outage, oversize file or message, or the recipient’s inbox hitting the storage limit. Soft bounces are usually outside the sender’s control. Most soft bounces resolve on their own after a short time. However, if they can’t be resolved, they become re-classified as a permanent delivery failure. In other words, they become a hard bounce.


Spam is unwelcomed email content that is potentially malicious or may open backdoors for hackers to lift sensitive personal details.


Spamming is the act of sending unsolicited messages or malicious content to recipients in bulk. This results in brands ending up on an email blocklist or in the spam folder, harming their IP reputation and inboxing rates. To combat this, brands can set up authentication protocols like SPF, BIMI, and DMARC, validating their domain names and protecting against bad actors.


Throttling is the practice of restricting the bandwidth of brands to carry out operations. In emails, throttling occurs when internet service providers (ISPs) limit the number of emails a brand can send for a specified period of time. During this period, a brand receives notices of temporary delivery failures, or soft bounces. This usually happens when ISPs detect that the brand’s IP address has suddenly ramped up its sending volume, which looks spammy. ISPs then throttle the domain while figuring out whether or not the domain is a legitimate sender.

Tracking pixel

A tracking pixel is a 1:1 pixel graphic generated by a line of code in an email. It tracks user behaviors and delivers personalized marketing to users as they browse the internet. Brands embed tracking pixels in either email headers or footers. Unlike cookies, which are highly noticeable and can be disabled, tracking pixels are virtually invisible to email subscribers.

Transactional email

A transactional email is automatically sent when triggered by user actions, like purchases or password reset requests. Transactional emails are also sent when a brand needs to share information with a subscriber, like order confirmations or payment received notifications. These are typically templatized and personalized since they’re only relevant to a particular user’s action. Because transactional emails are automated, most brands collaborate with an email service provider (ESP) like Mailgun, which sends the emails via an Application Programming Interface (API).

Transport Layer Security (TLS)

Transport Layer Security (TLS) is an encryption standard that protects emails and sensitive information during the sending and receiving process. It also authenticates the sender and verifies email content and data to ensure the data has not been tampered with.

Triggered emails

Triggered emails, also known as behavioral emails or transactional emails, are email messages sent automatically, based on user actions or specific events. They rely on customer data and behaviors. For instance, if a user purchases a product from a webshop, the brand will schedule an email automation trigger to send an order confirmation message.

Uniform Resource Locator

A Uniform Resource Locator (URL) is a unique string of text that points to a specific resource on the internet, like web pages or web domains. A URL includes a protocol (like HTTP or HTTPS) and domain or resource name, and sometimes a path, which indicates the specific location of a page within a website. All of these combined (without spaces) enable users to locate a web page. Here’s an example of a URL:

Unique clicks

Unique clicks are the number of users who have clicked on a link, counting only one click per user, even if the user has visited the link several times. They’re often measured based on trackers embedded in emails that determine whether clicks came from the same device or IP address.

Unsubscribe rate

An unsubscribe rate is the rate at which users unsubscribe from an email list. It’s a great metric to gauge an email program’s health, as it provides insights into user engagement – and whether subscribers are finding the content relevant. When demonstrated in a formula, the unsubscribe rate accounts for the total number of unsubscribes divided by the total number of emails delivered and multiplied by 100, expressed as a percentage. Here’s how:

User experience

User experience (UX) refers to a customer’s experience interacting with a brand and its products, website, and services. This includes everything from researching products to buying and using them. Brands design user interfaces (UIs) to create an optimal user experience and reduce friction as customers continue down the path to purchase.

User interface

A user interface (UI) is the graphical and clickable component of web applications that customers see and interact with. Web developers work with designers and other stakeholders to create an optimal interface for a business’ web application.

Variable cost

A variable cost refers to corporate expense that increases or decreases based on a business’ production or sales. It takes an upward trajectory as the company sells or produces more and vice versa. These are opposite the fixed costs that recur regularly in business operations, regardless of how much it produces or sells.


Webhooks are HyperText Transfer Protocol (HTTP) callbacks that push data from one application to another after a specific event occurs, like an email delivery. For example, if a company sends a newsletter to its subscribers but the email bounces back, a webhook can log the bounce, capture the data, and send a notification to designated recipients.


Now referred to in the industry as an allowlist or a passlist, a whitelist is a system used to allow only certain senders or messages to pass through a filter. All other senders or messages are blocked.

Whitelist email

Whitelisting emails means adding email addresses to an approved sender list. This informs email service providers to attempt to send email to these addresses, even if the email IDs are on an email bounce list. Companies whitelist email domains from certain senders to ensure they receive their emails. This signals inbox service providers (ISPs) to trust these email addresses and not place them in the spam folder.


A wireframe is a rough sketch of an interface, like an email template or website. Wireframes don’t usually use colors or graphics to convey the layout of the final product. Instead, designers use lines and shapes to depict elements of the final product, where lines indicate text and shapes denote images or graphics. These minimal mock-ups are named after the skeletal frameworks that look like they consist of “wires.”