An introduction to e-mail protocols

This article is an introduction to the POP3, IMAP, and SMTP e-mail protocols. If you plan on setting up a third-party client e-mail application like Thunderbird or Outlook, this article provides the background information you need.

Reading and sending e-mail

Any e-mail account provides two basic features: the ability to read and send messages.

The amount of configuration necessary to set up this functionality depends largely on the method you use to access your e-mail account. The webmail browser interface, for example, requires practically no additional configuration on your part. If you want to use a dedicated e-mail client like Outlook or Thunderbird, however, you must set up a way to access and read your incoming messages, as well as a way to send outgoing messages.

Incoming e-mail: POP3 and IMAP

To access your e-mail account using a third-party client application, you must set up a way to authenticate yourself and retrieve messages. The two most popular ways to do this are by using the POP3 or IMAP protocol. Using one of these protocols, you can access and download messages from an e-mail account on a remote server to your local computer. This is one of the main advantages of using a dedicated e-mail client application instead of webmail.

POP (Post Office Protocol) is currently in version 3, and commonly called POP3. POP3 is widely supported, and enables you to access an account, download and read messages, and do other related tasks. By default, POP uses TCP port 110, but there is also a secure SSL/TLS implementation that uses port 995.

IMAP (Internet Message Access Protocol) is similar to POP, but offers more functionality, particularly for mailbox synchronization and management. The default IMAP port is 143, but there is also a secure SSL/TLS implementation that uses port 993.

  • IMAP keeps messages stored on the server, which means multiple client devices can access email, make changes, and synchronize seamlessly. By default POP downloads messages, stores them on a local device, and then deletes them on the server. If you want to use multiple devices to access your email accounts, in most cases you should use IMAP instead of POP.
  • A2 Hosting strongly recommends that you use the secure versions of IMAP and POP3 whenever possible.
Outgoing e-mail: SMTP

POP3 and IMAP allow you to access an e-mail account so you can read and manage your incoming messages. SMTP (Simple Mail Transfer Protocol) enables you to send messages to other e-mail addresses. For an e-mail account to be fully functional, SMTP must also be working correctly. In other words, POP3 and IMAP provide “incoming” functionality, while SMTP provides “outgoing” functionality.

SMTP servers are the “post offices” of the internet, routing e-mail messages between senders and recipients. If the sender and recipient are valid and authenticated users, then messages are usually delivered normally. If a message is undeliverable, however, SMTP servers return the message to the sender, along with a reason why the delivery failed. For example, you have probably received a “message undeliverable” notification if you ever mistyped a recipient's e-mail address.

If you use webmail to access your account, you don't have to worry about setting up access to an SMTP server, because it is already configured for you. However, if you want to use a standalone client application such as Thunderbird or Outlook to manage e-mail, you must configure access to an SMTP server. Otherwise, the client does not know how or where to send messages.

By default, SMTP servers use TCP port 25. As with POP3 and IMAP, there is also a secure SSL/TLS implementation that uses port 465 or 587. Additionally, A2 Hosting SMTP servers support port 2525, which you can use if your firewall or ISP blocks port 25.

A2 Hosting strongly recommends that you use the secure version of SMTP whenever possible.

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