Email Glossary: 21 terms email user needs to know

Margarita K. | January 12, 2023
18 mins

Have you come across an email-specific term but drawn a blank? Well, you’re in the right place! If you want to know your APOP from your IMAP, your Cc from your Bcc, or are stumped by terms like ‘Backscatter’, stick with us.

The Origin of Email

Email came before the entity we call the Internet itself. It was crucial to the development of what’s known as ‘interoperability’ — where different computers exchange information and resources through local area networks (LANs), or wide area networks (WANs).

Originally conceived at the Massachusetts Institute of Technology (MIT), email was developed as a simple way to allow users of a computer to communicate with users on another computer. The success of email resulted in the development of the Internet.

Email Terminology

This list contains the most up-to-date email-related terminology. It’s arranged alphabetically should you want to skip to a certain topic, or start from the beginning for a comprehensive overview of email terms.


An email attachment is any type of file sent along with an email message. Including an attachment in an email is a simple way to share documents and images. One or more files can be attached and sent over to an email recipient. Attachments can include anything from photos and documents to zipped files, folders, mp3s, and more.

By the way, have you ever received an email where it says ‘see attachment’, but nothing has been added to the email? It’s a pretty common mistake, so no wonder some providers (like, for example, Namecheap’s Private Email) solved this issue. If you try to send an email that contains words such as ‘attachment’ or ‘attached’ and have not added any files, a warning notice will be shown.

Most email providers limit the size of the attachment(s) that can be sent, and the number of attachments allowed per email. When attaching a file to an email, keep your email client’s file size limit in mind. To get around file size limitation, you may want to share a link to a file place in Dropbox, or a similar file hosting service within the email body.


Base64 is a method of encoding and decoding. It’s used to convert binary data transferred over the internet into the American Standard for Information Interchange (ASCII) text format. The need for Base64 arose from the need to attach binary content to emails without encountering any problems during transmission given that the system of transporting email messages is designed for plain text only (ASCII). The trouble with ASCII is it’s difficulty with handling other languages and arbitrary files.

The internet as an information highway, yet, the path for email to pass through is a narrow space. Think of a ten-tonne truck trying to travel through a small tunnel that only small carts can pass through. It’s like getting a truck through the tunnel. How does it pass through? To solve this problem, the truck needs to be dismantled to pass through, and rebuilt on the other end of the tunnel.

This is exactly how it works when you send an email with an attachment. Data is encoded in a process called encoding the binary data and transformed into ASCII text (the type that can be transported via email without problems). Once the message reaches the recipient, the data is encoded and the original file is rebuilt. Base64 is one method of encoding arbitrary data as plain ASCII text.


Backscatter is a type of bounce-back email that can be received in case of failed delivery. However, it can also be used for spam or phishing. How? The spammer sends a malicious email and puts forged email addresses into the ‘From’ or ‘Reply to’ fields. The receiving server may accept a spam message but later discover that it can’t be delivered because the Return-path, From or Reply-to domains are forged. So, when the email is identified as spam by the recipient’s mailbox, it ‘bounces back’ to the forged email addresses that were added to the ‘from’ bar.


The majority of the email sent worldwide is spam. Fortunately, a blocklist (also known as a blacklist) exists to sort through legitimate email versus spam before it reaches your inbox. An email blocklist is a database containing known sources of spam mail that is used to filter and block spam emails. Without email blocklists, it would be hard to accomplish much via email. Inboxes would be flooded and the intense traffic of back-and-forth emails between servers would stall a lot of mail from reaching its destination.

Servers query blocklist databases in real time to ascertain the reputation of an IP address or the domain used to send an email. Email servers use the information in a blocklist to decide whether the email is coming from a reputable source to help them determine whether to accept or reject incoming mail. To query a blocklist, the IP address sending the email is checked against the spam database. If it’s included on the list, it is called a "known" source of spam and the email server stops the message from reaching the sender. It’s important to note that not all spam mail is on the blocklist, it may not yet have been reported as a ‘known’ source of spam mail.

There are several types of blocklists. The most common types are private blocklists courtesy of well-known names McAfee, Cloudmark, and Hotmail, there are also many public blocklists such as Spamcop and Spamhaus. Private blocklists tend to be stricter with their spam filtering. There’s no way of knowing why your or any other site features on private blocklists, you’ll only know when you begin receiving bounceback emails from those particular spam lists. Conversely, publicly available spam lists can be searched. People can check if their IP address is on a list and they have the ability to resolve the listing. A good source for checking whether your IP address is on an email blocklist is MX Toolbox.

For email providers cooperating with different blocklists is a great way to boost security. For instance, Namecheap’s Jellyfish tool communicates closely with different authorities and blocklists to receive and use the data to provide high-quality protection for the customers who use Namecheap Private Email and Shared Hosting email

Email Address Fields

When you send an email, you need to select the address that will receive it (that is what ‘recipient’ means). You can add them to one of three fields: To, Cc and Bcc. All three fields can send an email to several recipients in one go, however, they work slightly differently.

To: The "To" line is for the primary recipient. Put the email address here if the message is for this recipient’s action and attention only. This line is visible to all other recipients (Cc, and Bcc) by default.

CC: Stands for Carbon copy. Put email address(es) here if you plan to send a copy to other recipients in addition to those listed in the ‘To’ field. This is used if you don’t mind that recipients seeing who else is getting a copy, because the list of recipients is included in the header message. By the way, if you want to send a copy to some or all of your employees, it’s possible to address a group in the email (like Instead of adding tons of email addresses to group emails, you’ll only need to add one, and every employee gets it delivered to their mailbox.

BCC: Similar to Cc, Blind Carbon Copy sends a copy of a message for the information of a large number of people. It is used if you are sending someone a copy of an email, and you don’t want the other recipients to see that it was also sent to this email address because the message doesn’t include that information in its header fields. Bcc recipients can see those listed in the To and Cc list. However, if the recipients (who were added to the ‘To’ and ‘CC’ fields) reply, their emails won’t be seen by the ‘BCC’ recipient.

From: As you might have expected, the From header field contains the messages author. The authors email address is always included in the From field. You can also include a name to appear alongside your email address by updating the Personal Information settings in your email client. Including your name will not only look more professional to outsiders, it remove any ambiguity over who is sending the message.

Email Message

An email message is made up of three components:

Envelope: An email message has two addresses associated with it. Message headers are used by the email recipient to identify who has messaged them. The lines From: and To: however, aren’t enough to route an email message on to the intended recipient. Simple Mail Transfer Protocol (SMTP) servers use what’s called an envelope to route email.
Just like a normal mail envelope, an email envelope helps route the message to the intended destination. The envelope is abstract since it’s not something that an email user will ever see. To route an email, the sender's email program connects to their outgoing server and tells it the sender's email address and that of the recipient. This interaction is called the envelope. It then sends the rest of the message which includes the headers ‘To’, ‘From’, ‘Date’ and ‘Subject’ and the message body itself.
The envelope and the message arrive together at the recipients mail server. The envelope is often discarded at this point, and the message lands in the recipient's mailbox.

Body message: The body of an email is the sum of the message contents. It can include text, images, links and media including attachments.

Header: The final component of an email is the header which is arguably the most interesting part of an email. The header precedes the content (the body) of the message and comprises fields that contain information about the sender and receiver, subject and date fields. There’s also information about the route the message took to find the recipient. Each server that processed the email message is given an entry in the header, which helps to track a message’s origin if it looks suspicious.

Email Address

An email address is the electronic version of a real life postbox. It can send and receive email messages over a network of connected machines such as a local network not connected to the wider internet or the internet.

Email Services

Given the popularity of email, there’s an abundance of email services offering a whole list of services each with various levels of functionality. Even though most ISPs provide email accounts to their service users, email providers such as Google, Zoho, ProtonMail, and Namecheap Private Email are preferred by many options since they are more reliable and offer greater functionality, advanced security, and more storage.

Free Services: Free services are widely available from Microsoft (Hotmail) and Google (Gmail). Millions of users take advantage of free web email services as they are a great choice for personal purposes, and of course, don’t cost anything to use. However, when it comes to businesses, using a generic free email service is not the best solution. They often lack security, can use your personal data for marketing, but most importantly, they look less credible when clients receive emails from their generic addresses.  

Paid Services: Paying for an email service brings several advantages. For business users, having a professional email address such as will make your service look more credible. Even if you’re not interested in custom email domain hosting, paying for email services offers extras such as additional storage, professional collaboration tools, and advanced anti-spam protection.

Email Abuse and Spam

While the prevalence of email has increased enormously over the last few decades, it has also become a perfect environment for abuse. Given the enormous use of emails sent and received worldwide every day, they have become a target for abuse and spam. Think about how cost-effective sending large amounts of email messages is to many recipients online. For this reason, it’s never been more important to choose an email provider who guarantees reliable spam protection and use filters that sort legitimate email from spammers.

Spam, also known as junk email is when unsolicited messages are sent by email. To be classified as spam an email should have fraudulent or malicious content. These are often sent in bulk (the act of spamming) by people you don’t know. Many spam emails contain links to familiar websites but in fact lead to sites that are hosting malware of phishing sites. Spam is delivered in various ways. These messages are orchestrated by a spammer. A spammer is an individual or an entity (group of individuals or a company) that sends spam emails.

There are several types of fraudulent spam practices that email users and email administrators need to watch out for:

Email Phishing where private data is captured. The user clicks on an email designed to look the same as that of a trusted party, your bank for example. The website asks for login and personal information. The most common phishing email involves a message alerting you to a problem with a sensitive account. Users click the link, enter their login and password details in the fraudulent site, and by doing so , hand them over to the scammer.

Email Spoofing is when an email message is sent with a forged sender address. Most of us know spam when we see it, but seeing a strange email from a friend or recognized third-party can be quite disconcerting. Even if it looks like it’s come from a recognized place (spammers go to huge lengths to use the  appropriate email lingo to give their message authenticity), it doesn't mean your friend, bank, etc. has been hacked. Spoofing is commonly used in tandem with spam and phishing emails to mislead recipient over the origin of the message.

Email Worms: An email worm is a nasty piece of work. It distributes copies of itself in an email attachment. The infected emails are sent email addresses that the worm has harvested from files on the infected computer. This can amount to thousands of infected computers and many more compromised emails being sent.

Most of the time, people get a virus from their own family and friends, since they aren’t as vigilant with determining whether to open something from a familiar party.

Fortunately, there are some things you can do to protect yourself. The most effective method to prevent viruses from infecting your computer is to keep your security and antivirus software up to date so that it can catch all malicious activity. Being vigilant is equally crucial. Make sure that you are aware of the kinds of emails and attachments you receive so you can see when something looks out of place. When you know regular emails you receive, it’s important to treat every unusual email as a threat. For example, an email with a subject like: ‘Here’s a finance monthly report’ comes, but you are not responsible for finances. Such emails might also contain an attachment that indeed looks like a financial report, but actually contains a malicious file. Knowing emails you receive, or services you subscribed to help you minimize risks of getting viruses or data leaks. Another step to preventing malpractice is using email programs with built-in spam filters. A great example of an anti-spam tool is Jellyfish that comes along with Namecheap Private Email.   


You’ve probably heard of the terms surrounding data storage metrics, but most people aren’t clued up on what they actually mean. Let’s start with a gigabyte (GB). A GB is a unit of storage containing 1,000 megabytes (1 MB), or 1 Billion bytes to be exact. Each bite is a single unit of storage made up of 8 bits.

By the way, according to Lifewire analysis, the average size of an email is 75KB meaning 1GB of storage can contain approximately 13k emails.        

LDAP (Light Directory Access Protocol)

LDAP is a directory service protocol used for authentication of files, users, applications, devices, and other resources on the Internet (or a corporate intranet). It allows email and other programs on the internet to communicate with other directory services servers.      


List-unsubscribe is an optional string of text you can add to the header portion of an email. This option allows email providers and email clients to handle their unsubscribes. This email practice allows recipients to easily unsubscribe from an email list. Users click the text ‘unsubscribe’ which features as a button to effortlessly unsubscribe themselves from a mailing list. For publishers and marketers list-unsubscribe is an email best practice and should be adopted by legitimate mailers in all their email campaigns.

MIME (Multipurpose Internet Mail Extensions)

MIME, short for Multipurpose Internet Mail Extensions make it easy to send file attachments with emails. MIME is an extension of the original capabilities of the original email protocol, which allowed nothing but plain text to be sent. The introduction of MIME was an exciting time during the development of modern email. Suddenly people could exchange different kinds of data files across the internet (video, audio, applications etc).

MUA (Mail User Agent)

A mail user agent is a type of program that allows you to send and receive emails. In daily life we just refer to an MUA as an email program. There are two types of MUA, classed by how the programs are accessed:

Email Client: allows a user to access the emails stored on an email server or Mail User Agent (MUA) like Thunderbird or Outlook. Messages are downloaded from a folder when the MUA connects to the email server via POP, or through the more advanced IMAP protocol. Once a message is sent, the email client connects to the server using SMTP protocol.

Web-based Email: If you’ve ever sent a message via Gmail, Hotmail, Outlook, or any other online email account, you have used webmail. While both webmail and client mail is used to send and receive email, webmail differs from client mail because it’s is an internet based app that’s accessed through a browser. Additional software or downloaded applications aren’t necessary because all of the work is handled by remote computers.

Public Key Cryptography

Public key cryptography is a secure algorithm that allows people to exchange encrypted emails and files over an open network such as the internet. Two keys, a public key and a private key are used to secure the data transmitted. Each key performs a unique function, the public key encrypts data during transmission while the private is used to decrypt it once it lands at the recipient. Public key cryptography is used across the internet when exchanging information, most notably when websites have an SSL certificate they deemed secure. Emails on the other hand rely on using Transport Layer Security protocol (TLS), the successor of SSL, to encrypt email data.

RFC (Request for Comments)

Request For Comments (RFC) is a crucial to internet governance. An RFC is a document drafted by the Internet Engineering Task Force (IETF) and the Internet Society (ISOC). These bodies are the principal agencies for standard setting governing the internet. They are tasked with describing the specifications for a particular technology. Most RFC documents use a common set of terms such as "not recommended" and "must".

Once an RFC is ratified by the IETF and ISOC, it becomes a formal standards document. All current RFCs can be found in the RFC-editor. For example, RFC 5782, a document regulating DNS Blocklists and Whitelists.

S/MIME (Secure Multipurpose Internet Mail Extensions)

Is the standard protocol for sending secure email messaging. S/MIME authenticates an email sender using a digital signature and to protect privacy, they can be encrypted.


What does the subject mean in an email? The name says it all: it’s a short description of the message. When you receive an email, the subject is in a prominent position next to the sender’s name plus a preview of the body of the message depending on your email account. The best subject lines are concise and to the point. Keeping this short and informative will give people an incentive to open your email. Additionally, it looks better when the subject is filled in for both personal and business conversations.


Mailto is an HTML tag that makes the text clickable and leads to the email creation window where you can start writing an email to the address you clicked on.


Unicode has made it easier to exchange text files across the world. Unicode is the international coding standard that makes it possible to represent character and symbols from across the world including Western, Asian, Arabic and so on. Each letter or symbol is assigned a unique numerical value that applies to all devices, platforms and programs. This provision of setting up binary codes for script or text characters was the first of its kind, and provides a consistent way of encoding multilingual plain text to this day.

Mail Servers: ISP, POP3, IMAP & Cloud

ISP Hosting

Internet Service Providers (ISPs) provide mail servers that work for standard personal email communications. This elementary service isn’t for sending mass emails. ISP servers often limit the number of emails you can send. With ISP hosting, your emails are stored on the ISP's servers. To check your email, you will need to configure an email client to connect to the ISP mail server and download your messages. If your ISP has any issues, it won’t be possible for you to get to your email messages.


POP3 is an email protocol that downloads email from the mail server and stores it on your computer. It allows you to read your emails when you aren't online, but some (or all) of the emails downloaded may no longer be available on the server. This can result in confusion when emails are checked using multiple devices. For example, when you send an email from your cell phone, you might not be able to view it on your tablet.

POP3  can be used effectively by those who access their email from one personal computer, and back up their drive on a regular basis. Although it is possible to arrange the storage of your emails on the remote service of most ISPs and other email providers, downloading emails is a slow process if you have a significant amount of correspondence on the mail server.

IMAP/IMAP IDLE Email Hosting

The IMAP abbreviation stands for Internet Message Access Protocol. It works by keeping your devices constantly in sync with the mail server, but like POP3, emails can also be accessed without an Internet connection (providing the whole email message was downloaded before disconnection). For example, if a message is deleted by a desktop email program, the program will sync with the email server, and all of the other devices (tablet, iMac, etc.) will update to reflect the changes. If your business uses a central mailbox that multiple people need to access, or you need to be free to check email from multiple locations, IMAP protocol is your best choice for email hosting.

Meanwhile, IMAP IDLE is a protocol extension that helps the server send real-time updates to a client. With the IMAP IDLE, the new message can be synced immediately.


There are two ways to receive emails: via the protocols POP (Post Office Protocol), or POP3 and IMAP (Internet Message Access Protocol). While POP has been the most popular protocol (ISPs promote it as their default, and preferred method), IMAP is viewed as the most efficient way to handle email. Email hosts recognize that IMAP is a more sensible way to use email, irrespective of the fact that it requires more resources on their side.


Picture of Margarita K.

Margarita K.

Private Email Product Operations Specialist

More content by Margarita K.

Getting Started Online

For bringing ideas to life

From starting up to creating your site, here’s everything you need to make big splash online.

See more
Join Our Newsletter

Stay inspired

Get all the latest offers, articles, and industry news straight to your mailbox every month.

Need help? We're always here for you.