What are SMTP, IMAP and POP3?

Continuing our articles about fundamental Internet protocols, it is time we discuss the ones that facilitate email delivery. SMTP, IMAP, and POP3 are the three protocols used to send and receive emails throughout the Internet. In this article, we will discuss each of them, their purpose, and differences, so read to learn more!

This post includes:

• What is SMTP?
• What are IMAP and POP?

What is SMTP?

Standing for Simple Mail Transfer Protocol, SMTP is the primary protocol for sending emails and typically uses ports 25, 465, or 587. Like all the other networking protocols that exist online, SMTP standardizes the way an email is sent to a recipient. This standardization allows for email delivery regardless of hardware or software.

The main job of SMTP is to deliver an email to an email server. It does not place that email into the recipient’s inbox, however. That is the primary difference here between SMTP and IMAP/POP3. We will elaborate on that a bit later, but for now, let us find out how SMTP works.

1. Establish Connection - As with all other things online, everything begins with establishing a connection. SMTP uses TCP as its transfer protocol, so the first step is to establish a TCP connection between the client (sender) and server (recipient);
2. Data Transfer - Next, sending the necessary data. That includes the email header–which has the destination address and subject line, the email’s body, and any other components it may have;
3. Mail Transfer Agent - The Mail Transfer Agent (MTA) aims to verify the recipient’s domain and then locate it online by querying the Domain Name System (DNS). That way, the server can find the recipient’s IP address and figure out where to send the email;
4. Closing the Connection - The final step is for the recipient’s client to inform the server that the email has been received, after which the connection is closed. When a new email has to be sent, the sender and recipient server will open a new SMTP connection.

For example, a piece of mail in real life may jump from post office to post office until it arrives at the desired destination, and email can do the same. The process above can repeat several times before an email lands in its intended inbox. Another similarity between emails and physical mail is that an envelope is involved. The SMTP envelope is the digital counterpart of a paper one. It contains information about where the email is coming from and where it is going.

So, while SMTP handles the “pushing” of the email to the recipient server, IMAP and POP3 handle the “pulling” of the email to the email client.

What are IMAP and POP?

The Internet Message Access Protocol (port 143 or 993) and the Post Office Protocol (port 110 or 995) are the two networking protocols responsible for pulling an email from an email server to an email client. They are necessary to facilitate the transition of the email from the recipient server to the specific email client. There is one fundamental difference between the two of them, however.

On one hand, you have IMAP, which acts as an intermediary between server and client. That means emails on the server can be accessed from any device via IMAP, but an online connection must always be present. In other words, IMAP does not allow for offline access to emails.

However, on the other hand, you have POP3 (3 being the newest version of the protocol), which allows email clients to open emails offline. POP3 downloads the emails to the device the email client is on and then removes them from the server. As you can see, there is a rather significant difference, but they both have their merits.

While IMAP allows you to read your emails from any device–you just need an email client and your login info–the messages will still take up space on the mail server. If you are in a Shared Hosting environment, that space can sometimes be limited. POP3, however, removes the email from the server, conserving space and removing the ability to view your messages on multiple devices.

Depending on your needs, you should consider one or the other. Both of them will deliver emails to your inbox, you just need to ensure you are using the correct one. You can check our tutorials on how to set up some of the more popular email clients with either protocol here.

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