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.


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.

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.

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.


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.


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.


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.


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.

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 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 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 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 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.

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.

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.

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.


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.

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.

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.


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).

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.

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.

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.

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 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).

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.

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.

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.

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.