Email Protocol: Definition, Types, and How to Use It

Email Protocol: Definition, Types, and How to Use It

Read this guide to learn what email protocol is, how email protocols work, and which email protocol to choose for your email campaigns. 

What Is an Email Protocol: Definition

Email protocols can be defined as rules for sending and receiving emails by mail transfer agents. These rules can include different instructions that indicate whether to receive an email or send an error notification. 

How an Email Protocol Works

People exchange email messages with the help of email clients and mail transfer agents (servers). When a user creates an inbox, they become “attached” to a certain mail server.

When a user sends an email, first it gets to the server. The server analyzes who is the addressee and sends the email to the agent of the recipient. Then, the recipient’s mail transfer agent also analyzes the email and displays it in the inbox.

How email protocols work: User sends an email. The sender's server analyzes and sends the email (regulated by an email protocol). The recipient's server receives and analyzes the email (regulated by an email protocol). The email is delivered.

The email exchange between the servers is regulated by email protocols. Email protocols regulate different instructions sent by mail transfer agents: for example, whether to receive or decline an email, notify the sender that the email is delivered or send a notification about an error. 

The connection between mail transfer agents happens with protocols and via special “points” — ports. Thus, every email protocol has its ports.

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What Are the Types of Email Protocols?

There are three types of email protocols: POP3, IMAP, and SMTP. Let’s take a closer look at each of them.

Three types of email protocols: POP3, IMAP, and SMTP

POP3 Email Protocol

POP3 email protocol doesn’t save emails on the mail server; instead, it downloads their copies to the device: a computer, a laptop, or a smartphone. After that, the POP3 protocol deletes the original emails. Thus, if you accidentally delete the copies from your device, you won’t be able to restore them.

Also, you can view the copies at any time without internet access: it’s convenient if your internet access is limited. 


The POP3 protocol has a simple interface: if everything is in order, it returns you the “OK” status; if something’s wrong, it gives you the “ERR” status.


The POP3 protocol uses two ports: 

  • 110 is for receiving non-encrypted emails;
  • 995 is for receiving encrypted emails (encrypted emails help you avoid being hacked). 

The POP3 email protocol works in the following way:

  1. When a user receives an email, they connect to the transfer agent with the help of the 110 or 995 ports. 
  2. All the emails are downloaded to the user’s device;
  3. The user views emails and deletes unnecessary ones;
  4. The user gets disconnected from the transfer agent.

IMAP Email Protocol 

When you use the IMAP email protocol, the emails are not deleted from the server: you can read an email from any device that has internet access without downloading it.

The IMAP email protocol downloads the email to your device only if you click it. However, only the textual content will be downloaded to the device; if you want to view the attachments, you can do it in your email client and only online.


IMAP uses a flag system that marks emails as read, important, recent, etc. For example:

  • \seen — the email is read;
  • \answered — the email is responded to;
  • \draft — the email is not finished;
  • \flagged — an important or urgent email.


The 143 port works by default: it allows the user to receive not encrypted emails. The 993 port is used for a safe connection to email.

The IMAP email protocol works in the following way:

  • The receiver’s server tells the sender’s server that the former is ready to receive emails.
  • The sender gets authorized in the email client;
  • The sender’s server learns that the user’s got authenticated;
  • The user selects the folder they want to work with;
  • After that, the server learns what folder the user has selected and can start to interact with emails.

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The SMTP email protocol is used for sending multiple messages between the sender and the recipient. The reception of emails happens via a set of commands. 

You can’t insert an image into such an email: the SMTP protocol can transfer only the textual content. However, you can attach an embedded image to the email.

The SMTP email protocol establishes a connection with the sender before sending an email: this step helps to deliver the email faster. If an email can’t be delivered, the sender receives an error report. 


HELO: a command that initiates the dialogue between the two servers: the receiver and the sender. 

EHELLO: a command that shows that the sender’s server wants to use the advanced SMTP protocol.

MAIL FROM: a command that identifies the sender’s email address.

RCPT TO: a command that identifies the recipient.

VRFY: a command used by the receiving server to verify whether the user name is valid or not.

SMTP Ports 

The SMTP email protocol uses the 25 port by default. Also, it uses the 2525 port for non-encrypted emails and the 465 port for secure sending.

The SMTP email protocol works in the following way:

The recipient’s server checks the settings of the sender’s server. If the settings are correct, the server allows the sending. The sender’s server connects with the recipient’s server and sends the email.

If everything goes right, the recipient’s server confirms the successful delivery. If the email failed to be delivered, the server sends a notification about an error.

 HTTP server

This protocol is used for online data exchange. Although it’s not meant for email connection, it can be used to get access to the online version of the inbox.

HTTP server works in the following way:

In order to display the online version of the inbox, the browser sends a request to the server that keeps the HTML document of this page. 

After the server responds, the browser studies the HTML document and requests additional files if necessary: for example, scripts, tables of styles, and other additional resources. Next, the browser puts together all the content and displays it as a single web page.

Which Email Protocol to Choose?

The SMTP protocol is perfect for sending bulk emails. If it’s not your case, you’d better choose between POP3 and IMAP.

If the size of your mail server is limited, it’s reasonable to use POP3: your emails won’t take much place on the server and will be deleted as soon as they’re transferred to the device.

If you need to synchronize your email with other devices, we’d recommend you use IMAP.

Example of two email protocols working together: SMTP and POP3

Summary: What You Must Know About Email Protocols

Email protocols are rules for interactions between the sender and the recipient.

There are three types of email protocols: IMAP, POP3, and SMTP. HTTP is not an email protocol, but it can be used to display the online version of the inbox. 

The SMTP email protocol works well for bulk newsletters. POP3 is good for cases when the size of the server is limited. IMAP should be used when you need to synchronize the inbox with other devices.

EmailSoldiers Team
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