Simple Mail Transfer Protocol (SMTP): What Is It?


Simple Mail Transfer Protocol (SMTP) is an internet protocol that enables you to send messages from one email address to another. Whenever you type a message and click send, a lot happens under the hood to ensure it gets to the intended recipient. SMTP plays a major role in the process that this article will explain.


What Is Simple Mail Transfer Protocol?

SMTP is a set of rules that different email clients and servers use to exchange information. This is not the only email protocol in existence, but it is the most commonly used one. The two other major ones are the internet message access protocol (IMAP) and post office protocol (POP). The latter are used by client software to retrieve email messages from the server.


How Does SMTP Work?

The first step is to set up an SMTP server, a physical computer that will send and receive emails plus the necessary software. The sender specifies the server’s address, port number and login credentials in their email client. When the ‘send’ button is pressed, a connection is established between the mail client and the SMTP server. 

With an active connection, your email client gives instructions to the server to transfer the message, including the recipient’s address and the message’s contents.

The server’s mail transfer agent (MTA) checks whether the sender’s and receiver’s email addresses are on the same domain. If they are, the message is transferred instantly, as the target mailbox resides on the same server. Otherwise, the server will have to consult the domain name system (DNS) to find the IP address of the recipient’s domain. Once it finds the necessary information, the message is transmitted to the corresponding server and finally gets to the intended recipient.


What Port Is SMTP Using?

A port number is a way to specify which application will process data that arrives on a computer. To connect to an SMTP server, you’ll need both its IP address and SMTP port number.

Note that there are two main stages in email transmission:

  • Submission: The process of submitting a message to an outgoing email server.
  • Relay: The process of transmitting a message between two servers. 

There are multiple ports that work in different situations. The four main ones are  25, 587, 465, and 2525.

Port 25

This port was established back in 1982, which makes it the oldest one of the four. It is the standard one for relaying messages from one server to another. This port is commonly abused to send spam messages, so many internet service providers (ISPs) block it.

Port 587

This is the default port for email submission. You can use other ports for this purpose, but it’s advisable to start with 587 because that is its core purpose. You can submit messages securely with 587.

Port 465

This port was originally registered to submit messages securely. However, it was reassigned for a different purpose after a short stint in that function. Despite its re-assignment, many ISPs still support it for submission.

Port 2525

2525 is not an official port recognized by the Internet Assigned Numbers Authority (IANA). However, it’s still widely used for message submission, and most ISPs support it. It is a suitable alternative if 587 is blocked.


What Is ESMTP?

ESMTP stands for Extended Simple Mail Transfer Protocol. As the name suggests, it is an extension of SMTP. It works similarly and was designed for greater flexibility and functionality in exchanging email messages.

ESMTP is based on the original SMTP protocol but comes with additional commands. For instance, it includes commands for requesting a message to be re-sent after an unsuccessful delivery.



We’ve explained the SMTP protocol and how it’s used to send messages from one address to another. We also explained the differences between the various port numbers. This information will help you choose a suitable SMTP provider for sending emails, such as UniOne.


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