SMTP: Simple Mail Transfer Protocol- What it is, how it works And More

Summary: This blog is powered by Net2Secure, provides a descriptive guide to SMTP. It explains what SMTP is, how it works behind the scenes to send emails, and the roles of SMTP servers, ports, and commands. No matter if you are handling email campaigns or just excited about how emails are sent, this guide will assist you in understanding the technology powering your inbox.

Have you ever tried to know what happens when you press “send” on an email? How does your message travel across networks and appear in someone else’s inbox in a jiffy? Not yet, here’s the answer. It’s all because of SMTP, Simple Mail Transfer Protocol. 

SMTP refers to the standard communication protocol used to send emails from one server to another. Doesn’t matter if you are emailing a friend or operating a business campaign, SMTP acts as the invisible mail carrier, ensuring your message reaches the right recipient. 

In this blog post, we are going to cover a detailed guide about SMTP, what it is, how it works, and much more. So, without any delay, let’s get started.

Simple Mail Transfer Protocol is the cornerstone of email communication globally. As it plays a significant role in delivering and receiving emails, it is also an intricate protocol that consists of detailed technical processes.

What is SMTP?

As discussed, SMTP is the general system for moving emails across the Internet, and it’s often used for website-generated emails. Whether you click “send” on a contact from or trigger an email confirmation when you buy a product online, it acts silently in the background to verify that your message is delivered to its destination successfully. 

The system has evolved from sending plain text in its early days to handling modern email features. Currently, MIME enables emails to include attachments and HTML formatting. However, SMTP has some limitations: it can be unsafe to cyberattacks and doesn’t have built-in security, so it requires extra protection to keep emails safe. 

What is an SMTP server?

Same as all servers, an SMTP server refers to an application that offers a service to other applications within a network, known as clients. Generally, it handles the sending, receiving, and relaying of email. 

When you send a letter from city A to city B, it first reaches a local post office in city A; here, it gets processed and sent on to the post office in city B, which is in charge of sending it to its final destination. The same happens with SMTP servers, though instead of taking time, the process takes a few times. 

What is SMTPS?

SMTPS mainly makes emails safer by integrating solid security to regular SMTP.  It utlizes TLS encryption to secure email data when it travels across the Internet, same as HTTPS keeps web browsing safe and secure. 

It basically works through port 465 and keeps your confidential information secret from others. However, setting up security certificates can be tricky and may operate a bit slower, you can also prefer STARTTLS to get the same protection. 

How does SMTP work?

The simplest way to describe how SMTP works is to go over the sending process, the individual rules and commands that power it, and the errors you may encounter. Once an SMTP server is established, email clients can connect to and communicate with it. When the user presses “send” on an email message, the email client opens an SMTP connection to the server so it can send. The SMTP connection is built on something known as a TCP connection, which stands for Transmission Control Protocol. 

From there, the SMTP client prefers commands to tell the server what to do and transfer data, like the sender’s email address, the recipient’s email address, and the email’s content. The Mail Transfer Agent verifies to see if both email addresses are from the same email domain, such as gmail.com. 

Understanding the basic commands to control SMTP

• HELP or EHLO: This is a crucial command to start the entire email sending process. This command is used to start the conversation, and the server generally requires sending back a HELO command containing the domain name or IP address.

• MAIL FROM: After recognizing the command, the sender passes this command that represents who sent the email.

• RCPT TO: The following command follows thousands of response codes that represent where to send the email. It can come and go as many times as you like, depending on the number of people receiving the email.

• Data: This will trigger a data transfer between the client and server. All message content is pushed to the SMTP server, which responds with a 345 response code. The content of the message is sent to the server, which sends a period on its line to signal the end of the message.

• QUIT: After the email is dispatched, the client issues the QUIT command to the server, terminating the connection. If it has been successfully terminated, the server will respond with a 221 code.

• RSET: This command is sent to the server when the mail transaction needs to be aborted. It doesn’t close the connection, but it does reset everything and remove all previous data about the email and the parties involved. You will generally prefer this when there has been an error, like inputting the wrong recipient information, and the process needs to be restarted.

SMTP Ports

• Port 587:  It is the most often used port for secure SMTP submission using TLS. It is suggested for client-to-server communication, as it ensures the security of the email transmission.

• Port 465: Earlier used for secure SMTP, this port is no longer considered an official standard and is mainly not fit anymore. Many email providers have moved away from port 465 in favor of port 587. 

• Port 25: Port 25 is mainly used for SMTP relay between mail servers, not for email submission from clients. It is frequently blocked by ISPs for outgoing mail due to its regular use for spam and malicious activities.

• Port 2525: While not a designated SMTP port, it is occasionally utilized as an alternative for SMTP submission, particularly when port 25 is restricted or blocked. Numerous email services offer this port as an option for secure communication.

What is the difference between SMTP and POP or IMAP?

Post Office Protocol and IMAP only recover mail from a server; they pull mail from the server down to a mail client. SMTP, on the other hand, only communicates with servers to send outgoing email. There are good use cases for each. 

Post Office Protocol [POP]: POP collects mail and holds it until clients retrieve it. Rather than sending mail, an email client fetches new messages from the server, which are then delivered to the client. All messages are kept locally on the user's hard drive, and the server does not keep any copy. They are removed from the server after being downloaded locally.

Internet Message Access Protocol [IMAP]: IMAP refers to an email protocol for managing and recovering email from the receiving server. It is only concerned with retrieving messages, so as discussed, you cannot use IMAP to send mail. Not similar to POP, though, IMAP doesn’t delete the message from the server, and doesn’t exclusively place an email onto a local source. 

How Does the SMTP Protocol Differ from Other Email Protocols?

The key distinction among these protocols is that SMTP is the sole protocol for sending or "pushing" emails between two unknown email servers. POP and IMAP are methods for retrieving or "fetching" email from your email server. Consequently, POP and IMAP restrict email forwarding solely to authenticated email servers. It is not suitable for interacting with entities outside your network. 

Conclusion

SMTP indeed plays a significant role in making email communication possible. From the moment you press “send,” it works behind the scenes to transfer your message securely and effectively to the intended recipient. While SMTP alone doesn’t offer built-in security, protocols like SMTPS and STARTTLS help shield crucial data during transmission. 

Understanding how SMTP functions, along with its servers, ports, and command structures, can give you better insight into the world of email systems. Furthermore, knowing how it differs from POP and IMAP describes its unique responsibility in sending emails, whereas the others are mainly focused on retrieving them. 

No matter if you manage a website, operate a marketing campaign, or are simply excited by how your daily emails travel across networks, having a strong grasp of SMTP is crucial to understanding the backbone of digital communication.