What is SMTP?

Posted on: August 2nd, 2022
By: Tadeo Martinez

Have you ever stopped and wondered how things work when you send an email? Every time you send an email, your computer communicates with the email server of the person you’re sending it to. To do this, your computer uses SMTP. 

SMTP is a communication protocol used for exchanging emails between servers. It’s the backbone of email communications, allowing messages to be sent and delivered from one server to another. Without it, your emails would go nowhere.

Before diving deep into the SMTP providers, let’s first look at what SMTP stands for and how it works.

A brief overview of SMTP

SMTP stands for Simple Mail Transfer Protocol, the process behind the email flow on the internet. 

SMTP is a network protocol designed to handle all of the behind-the-scenes work of sending an email, including routing the message through the correct server and ensuring that it is delivered to the recipient’s inbox.

It is usually paired with IMAP or POP3 (for example, by a user-level application), which handles the retrieval of messages, while SMTP simply sends messages to a server for forwarding.

Most email clients—including Apple Mail, Outlook, Gmail, and Yahoo Mail—rely on SMTP to send messages from a sender to a recipient.

What is an SMTP server?

An SMTP server is an application used to send, receive, and relay outgoing emails between senders and receivers. It acts as a go-between for email communications, making it possible to send emails from one server to another. 

When you send an electronic mail (email), it first goes through the SMTP server, which then forwards it to the recipient’s mail server. The recipient’s mail server then delivers the email to their inbox. 

For example, when you use your email account to send an email to your friend, the message goes through your email provider’s SMTP server before it reaches your friend’s inbox.

Think of it as a post office: The post office (SMTP server) is responsible for taking your letter (email) and delivering it to the recipient (your friend).

Is an SMTP server the same as a normal server?

An SMTP server is a server that is responsible for handling outgoing email messages.  In contrast, a normal server simply stores and retrieves information.

It is similar to a normal server because it has its IP address and domain name. However, it differs from a normal server because it is specifically designed for email communication.

How does SMTP work?

To understand how SMTP works, it’s helpful to think of it as a conversation between two computers. When you send an email, your computer (the client) connects to the server and sends commands to initiate the message transfer. The server then responds with codes that indicate whether or not the message was successfully received.

Here’s a simplified example of how the process works:

SMTP uses the Transmission Control Protocol (TCP) as its communication protocol; the first step starts with a TCP connection between client and server. 

The client then sends the server a series of commands along with the actual information of the email: the headers, which include the sender’s and recipient’s email addresses; the body, which contains the actual message; and any attachments.

The system runs a program called a Mail Transfer Agent (MTA). It checks the domain of the recipient’s email address, and if it differs from the sender’s, it checks the Domain Name System (DNS) to find the recipient’s IP address. 

The end user alerts the server when the data transmission is complete, and the server closes the connection. At this point, the server will not receive any additional email data from the client unless the user opens a new SMTP connection.

In most cases, this process happens quickly and without any problems. However, if there is an error somewhere along the way, you may receive an error message indicating that your email was not delivered successfully.

Basic SMTP commands

SMTP commands are a set of codes that power the transmission of email messages between servers. The following are some basic SMTP commands you should be aware of:

HELO or EHLO (Hello): Introduce yourself and request an extended mode

MAIL FROM: Specify the sender

RCPT TO (Recipient To): Specify the recipient

DATA: Specify the body of the email

QUIT: end the communication between the server and the client.

RSET (Reset): reset the connection

Some examples of SMTP error codes

The most common SMTP error codes are:

420: Timeout connection problem

421: Service not available

450: Request action not taken

451: Requested action aborted

500: Internal Server Error

510/511: Bad email address

512: A DNS error

How is SMTP different from other email protocols?

You may have heard of other email protocols, such as POP3 and IMAP. These protocols retrieve messages from a mail server but don’t handle sending messages.

POP3 (Post Office Protocol 3) is the most widely used protocol for retrieving messages from a mail server. When you use an email client like Outlook or Apple Mail, POP3 downloads messages from the server and saves them locally on your computer.

IMAP (Internet Message Access Protocol) is another frequently used email protocol for retrieving messages from a server. Unlike POP3, IMAP keeps messages on the server and only downloads headers locally. This enables you to access your email from multiple devices without losing any of your messages.

What port does SMTP use?

A port is a virtual place in a network where data is received; it’s like the apartment number in an address. Ports help systems sort networking data to the correct applications. 

On the modern web, there’s not only a specific SMTP port. Instead, there are four common SMTP ports:

Port 25 is a popular port used for connections between SMTP servers.

SMTP uses port 465 with Secure Sockets Layer (SSL) encryption. In today’s email systems, TLS has replaced SSL, and this port is no longer used. 

Port 587 is now the preferred port for email submission and supports TLS encryption. Some ISPs block port 25 to prevent spamming, in which case you can use port 587, the preferred port for SMTP submissions.

Port 2525 is not a legitimate SMTP port but is a good alternative when standard posts are not available or blocked.

Common SMTP server providers 

Many SMTP server providers are available, each with features and pricing plans. The following is a quick rundown of a few popular SMTP providers:

SMTP: a valuable way to boost email communication

Effective communication is the key to success! SMTP is a key protocol in the email communications landscape. The role of emails and the SMTP servers is vital in any organization for its smooth functioning.

Emails are the most widely used mode of communication for any business in this digital era. 

Therefore, neither individuals nor organizations would want to compromise on email deliverability. Apple Mail, Gmail, and Outlook are among the most preferred platforms for one-to-one communication.

So, it is always advisable to invest some time in basic research about the types of SMTP servers available online before choosing the one suitable for your business or personal needs.


Have any questions or comments? Write them below!

Leave a Reply

Your email address will not be published. Required fields are marked *