IMAP vs. POP3 vs. SMTP: What Are the Differences?

April 14, 2022


Emails are still the most prevalent communication method in the business world. Email messages are transferred over the internet using email protocols, i.e., rules for routing and addressing email data.

This article will compare three popular email transfer protocols - IMAP, POP3, and SMTP. The guide explain how each of the protocols works and how they work together to enable email messaging.

IMAP vs POP3 vs SMTP - What are the differences?

What is IMAP?

The Internet Message Access Protocol (IMAP) is an incoming email protocol that allows email synchronization on multiple devices. Alongside POP3, IMAP is the most popular email retrieval protocol, supported by all major email clients.

How Does IMAP Work?

IMAP enables efficient multiple-client email management by keeping messages on the server until the user decides to delete them. This way, even after a client downloads an email from the server, emails are available for synchronization on other devices.

When an IMAP client communicates with the server and finds a new message, it downloads only the message header, which serves as a preview. The entire message is downloaded only when the user decides to open it.

IMAP is a two-way protocol, meaning that the user can manage the content of the email server. Instead of just downloading the messages, IMAP allows marking messages on the server as read, replied to, forwarded, or deleted.

IMAP mail retrieval process has three steps:

  • The email client establishes a connection with the mail server.
  • The client downloads message headers from the server.
  • If the user decides to read a message, IMAP retrieves the message from the server.

IMAP establishes the connection with the server using the TCP/IP transport layer. Once the connection is established, the protocol uses port 143 to listen to the server. For secure SSL/TLS connections, IMAP uses port 993. The two ports are the default values for IMAP, but the user can change them.

A diagram illustrating how IMAP clients and IMAP servers communicate.

What is POP3?

The Post Office Protocol version 3 (POP3) is the most used version of the POP. It is a simple incoming email protocol that moves the messages stored on an email server to the local email client. POP is among the oldest email protocols, designed at the time when users accessed the internet using dial-up and other temporary connection types.

How Does POP3 Work?

POP3 is a one-way protocol. When an email clients checks the server using the POP3 protocol, it downloads new messages and then they are removed from the server. This enables users to read and manage their messages while they are offline.

Since the POP3 communication channel is not bidirectional, further management of messages is not possible and other devices cannot download copies. The default setup to delete the messages from the server prevents filling up the storage space.

POP3 connection has four stages:

  • Authorization - the client provides credentials for accessing the mail server.
  • Transaction - the client downloads the messages to the local machine.
  • Update - the server removes the downloaded messages.
  • End - the connection terminates.

To connect the email client and the server, POP3 utilizes TCP/IP. By default, POP3 servers use port 110. POP3S (Post Office Protocol 3 over TLS/SSL) over port 995 is used for encrypted communication.

A simple diagram illustrating how POP3 clients and POP3 servers communicate.

What is SMTP?

The Simple Mail Transfer Protocol (SMTP) is an outgoing email protocol. Unlike the incoming email protocols such as IMAP and POP3, the purpose of SMTP is to enable the transfer of messages from the local machine to an email server.

How Does SMTP Work?

When a user sends an email message, SMTP uses the Message Transfer Agent (MTA) software to forward the message to the desired internet location. SMTP is also a message filter, controlling the number of messages sent in a certain period.

SMTP connects to the email server in three stages:

  • Handshake - SMTP client and SMTP server establish a connection.
  • Transaction - the client pushes the outgoing messages to the server.
  • End - the connection is terminated.

Like IMAP and POP3, SMTP uses TCP/IP to connect to the internet. The default port for SMTP transmission is 587, and this port also handles TLS encrypted communication. SMTP relaying, i.e., the SMTP-based communication between two mail servers is traditionally carried out using port 25.

How Do They Work Together?

For an email messaging system to function properly, incoming and outgoing email protocols must work together. The following section describes the path an email message crosses on the way to the recipient.

  • SMTP connects the sender's email client with the email server usually belonging to the sender's internet service provider (ISP).
  • Once the message is on the server, SMTP searches the message header for the recipient's email address and obtains the IP address of the email domain in the Domain Name System.
  • The message is then transferred to the recipient's SMTP server.
  • The SMTP server delivers the message to the recipient's mailbox.
  • When the recipient connects to their mailbox, they can use IMAP or POP3 to download the message.
A diagram illustrating how the email protocols work together to enable email messaging.

IMAP vs. POP3: Which One to Choose?

The two incoming email protocols, IMAP and POP3, are designed with different use cases in mind. Deciding which one to choose depends on your specific needs and the resources at your disposal.

Choose IMAP when:

  • More than one device needs access to the emails - IMAP keeps emails on the server, so multiple devices can receive messages at the same time.
  • Mailboxes need to be synchronized - IMAP synchronizes each registered client with the state of the mail server.
  • You use folders to organize emails - IMAP allows for creating a hierarchical structure on your server. POP3 does not support this feature.

Choose POP3 when:

  • Server storage space is insufficient - POP3 automatically removes messages once they are downloaded. On the other hand, IMAP can quickly fill up an email server with emails.
  • A lightweight solution is preferred - Due to constant synchronization between the server and the client, IMAP consumes more CPU and RAM than POP3. Therefore, POP3 is a less resource-demanding solution.
  • Privacy is a priority - Unlike IMAP, POP3 does not leave messages on the server where they can be subject to potential security breaches.

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This comparison article covered the three most important email transfer protocols - IMAP, POP3, and SMTP. We presented each of them separately, explained how they work together, and provided essential points of comparison between the two competing retrieving protocols.

If you are considering moving your email to the cloud, read about the importance of backing up your Office 365 data.

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Marko Aleksic
Marko Aleksić is a Technical Writer at phoenixNAP. His innate curiosity regarding all things IT, combined with over a decade long background in writing, teaching and working in IT-related fields, led him to technical writing, where he has an opportunity to employ his skills and make technology less daunting to everyone.
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