Email glossary

There are plenty of email terms you might have come across but draw a blank. If you want to know your APOP from your IMAP, have trouble distinguishing your Cc from your Bcc, and are stumped by the term Backscatter, stick with us. We’ll start with how email came about and how it’s used today. The rest of the page covers the most common email terms, all summarized in plain English.

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, in which 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 beginnings of the internet.

Recommended reading: How email works →

Email Terminology

The list below contains the most up-to-date email related terminology. The list is arranged alphabetically should you want to skip to a certain topic, or start from 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 to over to an email recipient. Attachments can include anything from photos and documents to zipped files, folders, mp3s, and more.

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 program’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.


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

Servers query blacklist 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 blacklist 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 blacklist, 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 blacklist, it may not yet have been reported as a ‘known’ source of spam mail.

There are several types of blacklists. The most common types are private blacklists courtesy of well known names McAfee, Cloudmark and Hotmail, there’s are also many public blacklists such as Spamcop and Spamhaus. Private blacklists tend to be stricter with their spam filtering. There’s no way of knowing why yourself or any other site features on a private blacklists, 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 blacklist is MX Toolbox.

Email Glossary

Email Address Fields

When you send an email, you need to select the address that will receive it. 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.
  • 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.
  • 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 structure
  • POP server: A Post Office Protocol is the internet standard. This protocol sends usernames and passwords in plain text over the DNS network leaving them open for interception by a third party.
  • APOP (Authenticated Post Office Protocol): This is an extension of the original POP protocol. It’s more secure than standard POP since it uses a shared secret -a password that is never sent directly, it’s always sent in encrypted form so that it can’t be intercepted by a malicious party. APOP is an improvement on POP but still suffers from some vulnerabilities that allow for room for attack.
  • SMTP server: SMTP is an acronym for Simple Mail Transfer Protocol. This is used for routing messages around the internet. Whenever you send an email, the SMTP server handles directing it to its final destination. Your host’s SMTP server will converse with other SMTP servers’ routed the messages from one server to the next until it reaches its destination server and delivers the email.
  • IMAP: The Internet Message Access Protocol is the standard email protocol for retrieving messages.

Email Services

Given the popularity of email, there’s an abundance of email services offering a whole host of services each with various levels of functionality. Even though most ISPs provide email accounts to their service users, dedicated email services such as Google and Thunderbird are preferred by many since they offer greater functionality, integrated communication services such as chat and games, 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 and many businesses use a generic free email service to contact their customers.
  • Paid Services: Paying for an email service brings several advantages. For business users, having a professional email address such as yourname@yourdomain.com will make your service look more credible. Even if you’re not interested in a custom domain, paying for email services offer extras such as additional storage, professional collaboration tools and app suites.

Email Abuse and Spam

We’ve mentioned the prevalence of email use over the last few decades, it’s the 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 use filters that sort legitimate email from spammers.

Spam, also known as junk email is when unsolicited messages are sent by email. More often than not, spam is from a commercial advertiser because it’s email is a more cost effective medium for the sender. To be classed as spam an email has to be not only commercial but have fraudulent or malicious intent. 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, 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.

    Fortunately, there are some things you can do to protect yourself from worms. The most effective method to prevent worms infecting your computer is to keep your security and antivirus software up to date so that it can catch all new viruses and worms out there. Being vigilant is equally important. Make sure that you are aware of the kinds of emails and attachments you recieve. Another step to preventing malpractice is using email programs with built in spam filters such as Hotmail and Outlook. These programs not only track emails and filter them for viruses, they will give you peace of mind that your computer is free from viruses or worms.

    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. If in doubt, check with the sender to make sure it was they who sent the message, or if you do become infected, make sure to inform them so that they can fix the problem before it does more damage.


You’ve probably heard of the terms that 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.

Email glossary GB

LDAP (Light Directory Access Protocol)

LDAP is a directory service protocol. This purpose is a directory service is to provide a set of records organized in a hierarchical structure, similar to how a building directory contains a list of occupants along with their suite number and phone extension.

LDAP allows email and other programs on the internet to look up information from a server by querying this hierarchy. Anyone can use this protocol to locate individuals, organizations, and other resources on the network such as files and devices, on the internet or a corporate intranet.


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: An 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.

PST (Personal Folders File)

The Personal Folders File (PST) is how Microsoft Outlook stores data locally. PST is a file format that can hold emails, notes, contacts, calendars, and basically any Outlook data.

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 Blacklists and Whitelists can be viewed here.

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.


The name says it all: it’s a short description of the message. When you receive an email, the subject is in 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 incentive to open your email.


Threadhijacking occurs when an individual or group of people comment on an original post on an online forum and deliberately go off topic. This practice can be carried out anywhere people can interact with comment threads online from a message board, blog comments section or Facebook post for example. They might ask a different question to the original question or suggestion on the first post of the thread.

For example a thread may begin "Where is the best Tapas in Madrid" and someone might divert the conversation by commenting "There’s tapas everywhere in Madrid, what’s really important is the best Sangria. Where can you find the best Sangria in this town?"

Threadjacking isn’t a crime but it’s good to keep in mind that derailing a conversation is bad internet etiquette since it undermines the point of the original post. To discuss a new topic, it’s polite to start a new thread.


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.

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