What Is SMTP? Everything You Need to Know about Simple Mail Transfer Protocol

Daisy updated on Mar 22, 2023 | Home > Knowledge Center

Emailing is one of the critical aspects of online communication. However, there are protocols in place to send one email to another. These protocols ensure safety and security and provide a viable system for file and content transfer.


One such emailing system is SMPT. So, what is SMPT? Moreover, how does SMPT even work? Today, we're going to analyze it and understand its key details. This includes its working and how it's important. So, let's dive right into it.

What is SMTP?

SMTP stands for Simple Mail Transfer Protocol, and it has been used since the early days of the internet to send mail from one server to another. It is still being used today for delivering email messages between servers, but it does not work very well with mobile devices such as smartphones and tablets.

So, a few of the most important things to understand about SMPT include:

  • SMTP—a network protocol that allows the sending of electronic mail.
  • It is used to send emails from one computer to another via a network.
  • The SMTP protocol was designed to be used on TCP/IP networks and was developed as part of the original specification for Internet email in 1982.
  • The name "SMTP" derives from its purpose: it facilitates the transfer of email messages between mail servers.
  • It is an extension of the earlier Simple Mail Transfer Protocol (SMTP) version 1 (RFC 821). It is a text-based protocol that uses port 25 by default, though other ports can be used as well.

But, if you describe it simply, then it's a mail delivery protocol. Hence, SMTP is a protocol for delivering email messages from one computer to another. Moreover, SMTP doesn't only facilitate the sending of email from one computer to another; it also regulates email delivery.

Hence, it's one of the oldest yet one of the most reliable emailing systems still working today. Besides that, it works flawlessly with other and latest emailing systems like POP and IMAP. This makes it an ideal combination of security, speed, and delivery system for clients like Outlook, Gmail, etc.

How does SMTP Work?

SMTP is a protocol for sending email messages. SMTP is an Internet standard that defines how email messages are transferred from one computer to another. SMTP is not a language, but it's a protocol. It is the most basic protocol used in sending emails.

It's a set of rules and guidelines that define how messages are transmitted via the internet and other networks. Therefore, it requires four key things to work properly, which are:

The SMTP transfer process starts with an SMTP client (usually an email client) connecting to an SMTP server using port 25, issuing a STARTTLS command if it wants to use TLS encryption, and issuing commands like HELO, MAIL FROM, RCPT TO, DATA, etc.

The connection between these two devices can be closed by disconnecting from port 25 or by issuing a QUIT command that terminates all data connections after the email is sent.

Defining SMPT Server

An SMTP server is a server program that receives email messages from SMTP clients and forwards them to the delivery agent for processing. SMTP servers are used to relay messages between mail servers and provide a service for sending an email.

An SMTP host is a computer with an Internet connection that accepts incoming emails from other computers, stores them, and makes them available to any other computer requesting a message. On the other hand, an SMTP client is a software that allows users to send emails through an SMTP server.

Why is SMPT Important?

SMTP is a protocol that defines how messages are formatted, addressed, and transferred between mail servers. The protocol was designed to be as simple as possible to make it easy for people to set up their own email servers without needing any special knowledge about the underlying computer technology.

The key guidelines of SMTP are:

  • Messages must be sent in ASCII format
  • Messages are limited to a maximum length of 65,535 bytes
  • All lines must end with a carriage return and line feed sequence (CRLF)

Therefore, it's not only a key regulation; it's one of the most important in ensuring a smooth mailing process from client to server and vice versa.


These are the key aspects that you need to know about emailing today. Therefore, it's an essential requirement of any emailing service to employ SMTP and execute its command properly. This is what allows thorough and smooth emailing throughout the global network.

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Updated by Daisy

Daisy is the Senior editor of the writing team for EaseUS. She has been working in EaseUS for over ten years, starting from a technical writer to a team leader of the content group. As a professional author for over 10 years, she writes a lot to help people overcome their tech troubles.

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