Transactional email glossary

Transactional email glossary

Trying to wrap your head around all things email delivery? Look up popular email terms in our glossary—you’ll be a pro in no time!

A/B testing (Split testing)
A method of testing different versions of an email message to determine which performs better. Creating different versions of an email usually involves changing one element of the email at a time, such as the subject line, Call To Action (CTA), or email content such as an image or text. All versions of the email in the test are sent out equally to a percentage of recipients and the best performing email is then chosen automatically or manually, and sent to the remaining recipients. Learn more about email split testing.
Account creation email
An automated email sent to confirm the creation of an account on a website or application. Account creation emails typically contain important information about the user’s account, information or instructions on next steps to take, and/or a verification link to verify the account. 

API (Application Programming Interface) is a software intermediary that makes it possible for two different applications to communicate. It gives a better user and developer experience by allowing you to automate the data transfer between applications and optimize the process. (See email API.)

Auto-responder or autoresponder
An automated email that is sent in response to a specific action. They are often used in response to incoming email messages, for example, in customer support settings where an auto-responder may inform the customer how long it will take for their issue to be looked at. Auto-responders are also used in response to an action taken on a website or app, such as when a new subscriber joins an email list or downloads a resource.

Automated email
An email message that is sent automatically based on a pre-defined trigger or action, for example when a user signs up for a service, makes a purchase or registers for an event. These emails are commonly known as transactional emails. (See Transactional emails.)

Automation platform
A software tool or suite of tools used to automate marketing tasks, business processes and workflows. An automation platform allows you to design and execute workflows and analyze and integrate data, reporting and monitoring. Automation is often used to streamline repetitive tasks to make them more efficient. Some examples of how an automation platform can be used for transactional email include integrating e-commerce platforms to send emails when an order is made, or receiving notifications on a specific app when an email activity occurs, for example, when an email hard bounces. Some popular automation platforms include Zapier, Make, Pabbly Connect and Integrately. Learn more about our automation platform integrations. 

BCC stands for Blind Carbon Copy. BCC is a field in an email where recipients' addresses can be added without the other recipients seeing the addresses. BCC is typically used when you want to keep someone in the loop without directly involving them in the conversation, or when you want to keep specific email addresses private. 

A list of domains and/or IP addresses that are known sources of spam or other malicious email activity. Email blocklists are used by Internet Service Providers (ISPs), Email Service Providers (ESPs), spam filters and businesses to block incoming messages for listed IPs or domains. Being listed on an email blocklist can have a significant impact on your sender reputation and deliverability. 

Blocklist may also refer to a list of recipients you want to avoid sending emails to. (See Suppressions.) 

Bounce rate
The percentage of emails that were undeliverable and returned to the sender. There are two types of email bounces, hard bounce and soft bounce, which can occur for various reasons including sending to an invalid email address, spam filtering, and full inboxes. A high bounce rate can have a negative impact on your email deliverability. (See Hard bounce and Soft bounce.)

Bulk email
The practice of sending a large number of emails to multiple recipients at the same time, for example, an email newsletter. In transactional email, bulk email can be used to optimize sending and save on resources by consolidating multiple emails into a single request. Learn more about MailerSend's bulk email endpoint.

The CAN-SPAM Act regulates spam and sets a national standard for sending commercial emails in the United States. It includes requirements for commercial message sending, the right for subscribers to unsubscribe, and penalties for violations.

CC stands for Carbon Copy. CC is a field in an email where recipients' addresses can be added so that they can receive a copy of the email that is being sent to the recipient/s. CC is typically used when you want to keep someone who is not directly involved in a conversation in the loop or to provide transparency. 

Click-through rate (CTR)
The percentage of all recipients who clicked on a link in an email. To calculate CTR, divide the number of clicks by the number of emails delivered, then multiply by 100.

Click-to-open rate (CTOR)
The percentage of recipients who opened the email that clicked on a link. To calculate the CTOR, divide the number of clicks by the number of opens, then multiple by 100.

CNAME, or Canonical Name Record, is a type of record in the Domain Name System (DNS) that maps one domain name (an alias) to another (the canonical name). This can be used when running multiple services from a single IP address, as each can have its own entry in DNS, but all actually point to the same IP address. CNAME records make it easier to manage and remember domain names, especially in larger network environments.

Customer Relationship Management (CRM)
A software application used to manage and analyze customer interactions and data. CRM systems usually store customer data such as their contact information, and communication and purchase history. This allows multiple people across teams access this information to provide a personalized experience, as well as seamless communication regarding specific transactions. 

Dedicated IP address
A dedicated IP is used by a single sender. Unlike shared IP addresses, which are shared by multiple senders, you’re the only person responsible for the quality and reputation of the dedicated IP. The better the quality, the better your email deliverability. Learn how to request a dedicated IP.

The ability of an email to reach the inbox of a recipient, rather than being rejected or sent to the spam folder by Internet Service Providers (ISPs). Email deliverability can be affected by a number of things, including your sending infrastructure, email authentication, sender reputation, email content, and engagement. 

Delivery rate
The percentage of emails that are successfully delivered to recipients’ mail servers, regardless of whether they end up in the inbox or the spam or junk folder. 

Denial-of-service (DoS)
A cyber attack that aims to disable a server by flooding it with multiple requests until it is unable to function properly.

DNS (Domain Name System)
The Domain Name System is a global directory of domain names matched with their underlying IP address of the server where the website is stored.

DNS record
A record that is kept on a DNS server and provides information about a domain such as the IP address that is associated with it. Among other things, DNS records help with email authentication and provide instructions on how requests for a domain should be handled. Learn more about DNS records.

DNSSEC (Domain Name System Security Extensions)
A set of protocols that secures DNS by adding cryptographic signatures to existing DNS records.

DNS server
The physical or software component of a DNS that stores the information needed to respond to DNS queries.

DomainKeys Identified Mail (DKIM)
An email authentication standard that prevents email spoofing by adding an encrypted DKIM signature to the email header of all outgoing messages. DKIM is an essential part of email authentication.

Domain Message Authentication, Reporting & Conformance (DMARC)
An email authentication standard that uses DMARC records to bind togather SPF and DKIM, and tell receiving mail servers what to do with unauthenticated emails according to policies you have set. DMARC also allows senders to utilize DMARC reporting which enables them to review their domain sending activity to prevent unauthorized use and troubleshoot sending issues. 

Drag and drop
A type of building tool used for emails, websites and more that allows users to create templates by dragging and dropping pre-built elements that they can then customize with their own colors, fonts, content, design and imagery. Learn more about MailerSend’s drag and drop builder.

Drip campaign
A series of automated emails that are sent to a specific group of recipients over a set period of time. Emails are pre-written and set up as part of an automation with the goal of nurturing and engaging with the audience. Drip campaigns are usually educational and aim to provide value while driving recipients towards performing a specific action, for example, a welcome email campaign might have the goal of getting the recipient to complete the set up of their account or upgrade to paid version. 

EHLO stands for Extended Hello, a command used in the Simple Mail Transfer Protocol (SMTP) to initiate a session between the sending and receiving mail servers, and check the capabilities of the server.

Email alias
An alternate email address that is associated with the same mailbox as another email address. This allows the user to use multiple email addresses to receive emails to a single email account. Email alias’ are typically used to simplify email management by organizing incoming email messages depending on the email address used. 

Email API
A software intermediary that provides the functionality of an email service provider to other applications via code. It provides a set of programming instructions and protocols that allow for direct integration of an Email Service Provider’s (ESP) infrastructure into other websites or apps. Email API has all the features of SMTP and more, allowing for more efficient email sending, better personalization features, and improved server-side feedback. Learn more about MailerSend’s email API.

Email automation
The use of software to automate the sending of email messages based on specific triggers or actions. Email automation enables email senders send timely, personalized emails and increase engagement with individual subscribers or customers without the need to manually create each message. Some examples of email automations include welcome email sequences and educational email sequences. 

Email blast
A mass email, typically promotional, that is sent to a large number of recipients at once.

Email client
A desktop, mobile or web software application used to access and manage a user's email, with features that allow them to send and receive emails, manage contacts, use multiple accounts, and filter messages. Examples include Microsoft Outlook, Apple Mail, and Gmail.

Email encryption
The process of converting plain text into coded text in order to protect the email's contents from being read in the case of interception, so that only the intended recipient can read and understand the contents. Some encryption protocols include Transport Layer Security (TLS) and Secure/Multipurpose Internet Mail Extensions (S/MIME).

Email hosting
A paid service that provides email accounts and servers for businesses and individuals. Email hosting allows users to use their own domain name so that they can create custom email addresses unlike free services such as Gmail, Outlook or Yahoo.

Email list
A list of email addresses that a business or individual uses to send emails to. In the case of transactional email, emails may be added to an email list when a user or customer creates an account, makes a purchase, or performs another action. In email marketing, subscribers are added when they signup to receive newsletters, offers or news, or register for a resource or lead magnet. 

Email marketing
The practice of sending promotional messages or offers to a targeted audience via email. Some types of email marketing campaigns include newsletters, special offers or discounts, sales, surveys, and product launches. It’s important to remember that email marketing allows for promotional messaging while transactional email does not. Learn why you should keep transactional and marketing emails separate. 

Email personalization
The process of customizing email content or subject lines using recipient data including their personal information or account activity. Email personalization can include the use of personalization variables to dynamically insert data, such as the recipient’s name, address or past purchases, or segmentation to send targeted content based on previous account/purchase activity, in the case of email marketing. Learn more about personalization variables. 

Email relay
Like a relay race, an email relay describes the process of handing an email from one server to another until it reaches the finish line—the recipient’s inbox. (See SMTP relay).

Email server
The hardware or software that manages the delivery and storage of email messages. The email server is responsible for initiating, processing and delivering email messages to their intended destination. 

Email signature
A block of text or an image that is automatically added to the end of an email. Email signatures usually include a sign-off, the sender’s details, social media links, website links, contact details etc. 

Email template
A pre-designed layout or format for an email message that can be populated with dynamic content or updated manually before sending to recipients. Email templates allow businesses and individuals to save time on creating emails and maintain consistency in the design and branding of their messages. Learn about MailerSend’s dynamic email templates.

Email tracking
The use of a unique tracking code or pixel in an email to track activity including delivery, opens, clicks, unsubscribes, spam complaints, and rejection. Learn more about email tracking.

Email validation
Email validation, also known as email verification, is the process of checking to see if an email address, or a list of email addresses, is valid and safe to send to. Sending to invalid email addresses can negatively impact your sender reputation and deliverability, so it’s important to regularly clean your email list. Learn more about email validation. 

A specific URL or web address that acts as the touchpoint of an API where two applications communicate with each other. An endpoint is typically used to interact with or retrieve data from a server or web app. APIs have many endpoints which are defined by the API provider, for example, to send emails, fetch email activity, and so on. Check out MailerSend’s API documentation to learn more about our endpoints.

Hard bounce
A type of email delivery failure usually caused by a permanent error, such as a non-existent mailbox, which results in the email being returned to the sender. Once a hard bounce has occurred, email senders should take steps to prevent further messages from being sent to the email address that bounced, as this can impact sender reputation and email deliverability. 

HTML email
An email that is created using HTML, allowing for the use of additional formatting capabilities, images, and links.

HTTP method
Standardized commands used to request actions that you can perform with a REST API. HTTP methods include GET to retrieve data, PUT to replace data, POST to submit data, and DELETE to remove data.

Identifier alignment
A DMARC process that compares the sending domain with the “From” sender address.

Internet Mail Access Protocol (IMAP) is a protocol used by email clients to retrieve email messages from email servers. 

IMAP client
A desktop, mobile or web-based program that enables users to access and manage email messages from a mail server using the IMAP protocol. Some popular examples of IMAP clients include Microsoft Outlook, Apple Mail, Mozilla, Thunderbird, and Gmail

IMAP server
A type of server that handles the retrieval of email messages for client email programs using IMAP. As messages remain stored on the server even when an email client downloads it and displays it, it allows for any changes to the email to be also stored on the server, so that they can been seen across multiple devices or clients. 

The latest version of the IMAP protocol, which includes additional features such as metadata and UTF-8 support.

Inbound routing
Inbound email or SMS routing is when an email service provider (ESP) receives email or text messages on your behalf, parses them and then integrates them in your application. This allows users and customers to add comments, post replies and respond to emails which will then be integrated into your application. An example of this is support emails being routed to a CRM or support platform so that all customer service agents can access and respond to tickets, and the customers’ history can be logged. Learn more about inbound routing.

Internet Service Provider (ISP)
An ISP is a general term for businesses that provide services related to the Internet. Some examples include high-speed broadband services (Verizon), mailbox services ( and web hosting services (GoDaddy).

IP address
A unique numerical address that is assigned to every device on the Internet.

IP allowlist
A list of IP addresses that are authorized to access and use a service, such as SMTP credentials and API. Learn more about IP allowlisting.

Stands for JavaScript Object Notation, a lightweight data format used to exchange data between a web server and a client.

List hygiene
The process of regularly cleaning and updating an email list to ensure that it only contains active and engaged subscribers. Email senders should practice good email list hygiene to remove inactive or invalid email addresses as these can have a negative impact on sender reputation and email deliverability. 

List segmentation
The practice of segmenting an email list into smaller groups based on different criteria such as demographics, behavior or interests. Email list segmentation allows businesses and individuals to personalize email content for the needs and interests of their recipients.

Mail client
A software program or web-based service that enables users to send, receive and manage email messages. Some examples of mail clients include Gmail, Microsoft Outlook, Mozilla, and Apple Mail. 

Mail server
A computer program or application, hosted on a physical server or virtually, that manages the delivery and storage of email messages for one or more email domains. Mail servers communicate with each other to send and route email messages to their intended recipients.

MDA (Mail Delivery Agent)
A software component that receives email from a mail server and delivers it to the intended recipient's mailbox. An MDA is the intermediary between a mail server and a mailbox’ they work with the Mail Transfer Agent (MTA) to route email messages between servers. Examples include Postfix and Exim.

MSA (Mail Submission Agent)
Also known as a Message Submission Agent, it describes the software that receives email from a Mail User Agent (MUA) and works with a Mail Transport Agent (MTA) to deliver the email.

MTA (Mail Transfer Agent)
A software component, also known as Mail Transport Agent, responsible for transferring email from one server to another. Examples include Sendmail and Postfix.

MUA (Mail user agent)
Another name for an email client (See Mail client), referring to the software that is used to access and manage email messages on a device or computer.

MX (Mail Exchanger) records
MX records are DNS (Domain Name System) records that specify which server is responsible for handling email for a particular domain. They are used by email systems to route email to the correct server. Learn more about DNS records.

Notification email
An email sent to inform the recipient of important information or updates related to their account or purchase. Notification emails are usually generated and sent automatically, triggered by a specific event. Some examples of notification emails include shipping updates, account status updates, and support ticket updates.

Open rate
A metric used in email to measure the percentage of recipients who opened an email message. Open rate is calculated by dividing the number of email opens by the number of emails delivered and then multiplying it by 100. Open rate can be used to measure the engagement of recipients and how successful a particular email subject line is.

The process of obtaining permission from an individual to send them emails. For transactional emails, opt-in consent is not needed, and is implied when a user signs up for an account or service. This is because transactional emails fall under the lawful base of legitimate interest. For email marketing, however, opt-in consent is needed before any promotion content or newsletters can be sent. 

Order confirmation
A type of transactional email sent to a customer to confirm an order, usually online. Order confirmation emails usually contain a delivery address, order number, items purchased, and estimated shipping time. 

Password reset
A type of transactional email sent to a user when they request to reset their password. When the user clicks on a button or link to reset their password, they’ll be prompted to enter their email address. If the email address matches a user account, a password reset email will be sent. The email will usually contain instructions and a unique URL that will allow the user to create a new password for their account. 

Person-in-the-middle (PITM)
An attack where hackers intercept emails on their way to recipients to eavesdrop or change their contents.

An attack used to steal your personal information by sending emails that impersonate websites that you already use. Phishing emails may ask you to click on a link, provide personal details, login to your account, or download an attachment that contains a virus or malware.

Post Office Protocol (POP) is a protocol used by email clients to retrieve email messages from email servers.

POP client
A desktop, mobile or web-based program that enables users to access and manage email messages from a mail server using the POP protocol. Some popular examples of POP clients include Microsoft Outlook, Apple Mail, Thunderbird and Opera.

POP retrieval
The process of retrieving email messages from a POP server on an email client using the POP protocol. When emails are downloaded from a POP server, they may be deleted. This means that if you access your emails on a POP server from multiple devices or applications, they may each show different emails. Some POP email clients, however, are configured to leave a copy of the email on the server.  

POP server
A type of server that handles the retrieval of email messages for client email programs using POP. As messages are often deleted from the server when an email client downloads it and displays it, emails and changes to emails may not be displayed the same across multiple applications or devices. 

The latest version of the Post Office Protocol (POP), which is used for retrieving email messages from a mail server. POP3 is the most widely used version of the protocol. 

An email preheader is displayed next to the subject line of an email in the inbox. It allows senders to expand on the subject line and give recipient’s additional information about the content of the email. 

Reporting URI for aggregate data (RUA)
A DMARC tag that contains an email address to receive aggregate reports of email activity.

Reporting URI for failure data (RUF)
A DMARC tag that contains an email address to receive forensic reports of emails that failed authentication.

Representational State Transfer is a software architectural style that uses web standards like HTTP to create interactive web applications.

Rich Text
A format that allows users to create Rich Text content. Traditionally, Rich Text allows used to exchange documents with their formatting between different word processors. In email, Rich Text allows you to create plain text type emails with additional formatting capabilities of HTML, such as bold, italicized, underlined text, fonts, links and images. Learn more about creating Rich Text emails. 

An SDK, or Software Development Kit, is a set of tools provided by the developer of a platform to allow for easy integration or development of applications. It allows user to use the platform’s framework, or the platform itself. 

Secure Sockets Layer (SSL)
SSL is a cryptographic protocol used to secure communication between computers on a network. It was replaced by TLS in 1999. (See TLS).

Sender ID
A historical email authentification method for Microsoft Exchange that listed the IP addresses of mail servers authorized to send emails from a domain.

Sender Policy Framework (SPF)
An email authentication standard that lists the sending IP of mail servers that are authorized to send messages from your domain. SPF is implemented as a TXT record on the DNS of the sending domains and is an essential protocol for email sending. 

Sender reputation
The reputation of the sender as determined by Internet Service Providers (ISPs) and spam filters. Sender reputation is a measure of credibility and trustworthiness as an email sender and  can affect emails are delivered to the inbox or spam folder. It’s based on numerous factors including sending behavior, email engagement, email content, and blocklist appearances. 

Shipping confirmation
A type of transactional email sent to a customer when their order has been shipped. Shipping confirmation emails usually contain the shipping address, courier, tracking number, and estimated delivery date. 

Simple Mail Transport Protocol (SMTP)
A communication protocol used to send and receive emails over the Internet. SMTP is the standard protocol for sending emails. 

SMTP authentication
A process used to verify the identity of an email sender, typically via a username and password using an authentication method like PLAIN, LOGIN or CRAM-MD5. SMTP authentication prevents the unauthorized access and use of an email account which helps reduce email abuse such as spam and other cyberattacks. 

SMTP error codes
Standard codes used by email servers that indicate the status of an SMTP email delivery attempt. SMTP error codes are three-digit numbers that refer to a specific error or delivery status. They can be used to troubleshoot email delivery issues. Learn more about SMTP error codes.

SMTP port
A logical address that identifies a specific network service. When a computer program wants to communicate with a network service, it will connect to the relevant port to do so. Learn more about SMTP ports. 

SMTP relay
An email delivery service that routes messages through email servers based on the industry-standard Simple Mail Transfer Protocol. Learn more about MailerSend’s SMTP relay.

SMTP server
SMTP (Simple Mail Transfer Protocol) is the protocol for delivering outgoing emails.
SMTP relay servers can be used to send bulk emails to mail servers. SMTP servers are trusted third parties that provide the technology needed to relay email over SMTP, and can help you deliver high-volume sending and transactional emails.

Soft bounce
A type of email delivery failure caused by a temporary error, such as a full mailbox, which results in the email being returned to the sender. Once a soft bounce has occurred, email senders should take steps to track any further messages that are sent to see if the bouncing of messages persists. If emails to a specific email address continue to soft bounce, you should take steps to exclude them from your sendings, as this can impact sender reputation and email deliverability.

Unsolicited, unwanted, or irrelevant email. Spam emails are typically sent on mass to large numbers of recipients who have not consented to receiving emails from the sender. As well as usually being promotional, Spam emails may often be used for phishing attempts or contain fake or misleading content. 

The practice of sending unsolicited emails in bulk to a large number of recipients.

Spam filter
A tool or algorithm used by email service providers and email administrators to identify and block unwanted or suspicious email.

A fraudulent practice where spammers forge your domain or sender name to send fake messages that appear to come from you. Spoofing can occur in 3 ways: with a lookalike sender name, a lookalike domain, or forged email header details to display your actuall domain. Learn more about email spoofing. 

STARTTLS is a command sent from an email client to an email server. It informs the server to upgrade a previously insecure connection into a secure one.

Subject line
The text that appears in the subject field of an email which is used to summarize the email's content and entice recipients to open it.

A list of email addresses that you have removed from your recipient list and are refraining from sending to. Suppressions lists usually contain email addresses that have hard bounced, unsubscribed, made spam complaints, or are in some other way invalid. Learn more about email suppression lists.

Transactional email
Automated emails containing information that is important or relevant to an action that a user or customer has performed on your website or app. These emails are triggered by an event, such as an e-commerce order, account signup, or password reset request. Learn more about transactional email. 

Transactional SMS
Automated text messages containing information that is important or relevant to an action that a user or customer has triggered on your website or app. Examples include 2FA, appointment reminders, and shipping notifications. Learn more about Transactional SMS.

Transport Layer Security (TLS)
A cryptographic protocol that is used to encrypt and secure data that is sent over the Internet.

Specific events or actions that cause an automated email to be sent, or another event to occur within a workflow. (See Workflow).

Two-factor authentication (2FA)
An authentication process that requires two methods in order to access an account or system. This usually involves something the user knows, such as a username or email address and something they user has, such as access to an email account, mobile device or a physical security token. 

The process of removing oneself from an email list or opting out of receiving certain types of emails. 

Webhooks enable real-time notifications about various events without having to constantly poll the API. It uses HTTP callbacks to listen for events at your URL endpoint so that you can automatically trigger a reaction.

A web-based email client that allows users to access and manage their email messages through a web browser.

The practice of adding an email address or domain to a recipient's safe senders list, ensuring that emails from that sender will be delivered to the inbox.

A series of automated actions or tasks that are triggered by a specific event or action. For example, a workflow can be used to update a database or spreadsheet when a user creates an account.

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