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What is SMTP?

Email with arrow

When you send an email, your email client and multiple servers on the internet use the Simple Mail Transport Protocol or SMTP to move that message around the internet and ultimately into the recipient’s mailbox.

There are two main types of SMTP servers: relays and receivers. Relays accept email from users and then route it to the recipient. Receivers accept mail from relay servers and deliver it into a mailbox.

When you click “Send” in your email program, the program connects to the relay server configured in its settings. This setting is often called an “Outgoing Email Server.” The relay will then use DNS (domain name service) to look up the receiver email server for the recipient address in the email. Every domain that wants to receive email, should configure a DNS Mail Exchanger or MX record, which defines where the receiver server is for that domain.

The relay will then connect to the receiver server and will use SMTP to deliver the message. Then the receiver will then store the message in the recipient’s mailbox for later retrieval.

What is POP3?

Post Office Protocol version 3 or POP3 is a simple protocol for accessing mail. When accessing email via POP3 unencrypted, it uses port 110. When encryption is in place, the default port is 995. This port is set in your email client’s settings. This setting is often called an “incoming mail server.” POP3 downloads all the messages from the server and stores the messages locally on your computer or device. One advantage of this is that email is available to be read offline. This also uses less storage on the email server.

However, a disadvantage is that it is complicated to access email using POP3 on multiple devices. Some email clients have a setting that allows you to save a copy of the message on the server so that you can download it again on another device. Because POP3 works best with one device and typically does not keep the messages stored on the server, you will need to backup your computer or device.

If your computer dies for any reason, all of your emails will be lost. Additionally, POP3 does not have a good way to understand and use folders. This means your sent mail, trash, and other folders are all stored locally on your computer. If you are using multiple devices, their sent mail, trash, and other folders will most likely be different from each other.

What is IMAP?

Another, more advanced method to retrieve mail from your email provider’s servers is using Internet Message Access Protocol or IMAP. IMAP is a protocol your email client uses to synchronize all the messages between the server and your clients. With IMAP, the email is stored on the server. You can access your inbox, sent mail, drafts, and other folders using an IMAP client. Those folders are stored on the server as well.

When using an unencrypted IMAP connection, the default port is 143. It is highly recommended that everyone use the encrypted IMAP port, which defaults to 993. Your email provider may use different ports. Another fact about IMAP is that it only downloads the necessary information about each message so that the client can display that for you. This information includes things like the sender, subject, and date when it was sent. IMAP does not download the entire message until you open the message, which allows the initial connection and startup faster.

Another advantage of IMAP is it allows you to synchronize email across multiple devices. When you read a message on your personal laptop, it will also show up as read on your work computer, smartphone, tablet, and other devices. Because everything is stored on the server, IMAP can be challenging to use on slower connections. It also may be impossible to read your email if you do not have an internet connection at all.

Email Ports

When sending an unencrypted email, port 25 is used. When sending encrypted email, port 465 is used. The preferred port is port 587, which can start as unencrypted and upgrade the connection to encrypted if possible. Now that the message is stored in the recipient’s mailbox, there are two ways that the user could retrieve the message.

Which Protocol Should I Use?

When sending mail, you should use SMTP. That is the worldwide standard for sending mail. If you have a reliable and fast internet connection, then IMAP is probably the way to receive an email. If your connection is not fast or reliable, then POP3 may be the way to go. Also, keep in mind that POP3 is not ideal for multiple devices.

How Does the Entire Process Work?

First, we will offer a scenario in which Bob will send a message to Jim. Jim only has one laptop and has a slower internet connection, so he has chosen to use POP3. The exchange looks like this.

When Bob hits send, his client will connect to his SMTP relay. His relay will look up the DNS MX record for the email domain of Jim. This will tell the relay server to connect using SMTP to Jim’s email receiver server. Jim will then use POP3 to download the message to his computer.

In the next scenario, Bob will email Sara. Sara is on the go and has a fast and reliable internet connection on her laptop and smartphone. This exchange would look like this.

The first part is identical to Bob’s email to Jim. He hits send, and his client uses SMTP to connect to his relay server. His relay server does a DNS MX record lookup to determine Sara’s domain as an email receiver on her server. It then connects via SMTP and delivers the message to Sara’s inbox. Sara uses IMAP, which communicates back and forth with her receiver server. She can do this both on her laptop and on her smartphone.


Email is a constant in today's world and a necessary part of every business. Understanding how it operates and the role it plays allows us to take full advantage of all its functionalities. It keeps us in contact with our customers, employees, and other businesses. Email plays a vital part in sharing information. and keeping lines of communication open.

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About the Author: Matt Holtz

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