SMTP: What Is It and How Does It Work?

Most people send and receive emails without even knowing what’s going on behind the scenes to deliver their messages. The email delivery process is enabled by various email protocols, which are a set of rules that enable email clients to communicate with each other. Simple Mail Transfer Protocol (SMTP) is one of the most popular protocols, and this article will show you what it is and how it works.

What Is SMTP?

The Simple Mail Transfer Protocol, or SMTP, is the most popular communication protocol for sending and receiving emails. All popular email clients, e.g., Gmail, Yahoo Mail, and Apple Mail support it.

SMTP was conceived in the 1980s and has been updated and modified many times. It is a segment of the application layer of the TCP/IP network model.

What Is an SMTP Server and What Is It for?

The SMTP server is a software application that exchanges emails by implementing the SMTP protocol. Think of it as a physical post office. When you want to send a letter from City A to City B, you’ll first go to a post office in City A and hand over the letter. It’ll then be processed and sent to the post office in City B. SMTP servers are the post offices in this case, but they are handling emails instead of physical letters. It also takes mere seconds to process messages instead of days with physical post offices.

How Does SMTP Work

The first step for this protocol to work is to set up the server. Once it is up and running, email clients can connect to it and use it for communication. Once the end user hits the email “send” button, the client initiates a connection to the server. Then, the client uses technical commands to instruct the server on what to transfer (the message content) and where to transfer it (the sender’s and recipient’s email addresses). The Mail Transfer Agent (MTA) checks to see if both addresses are hosted on the same domain name. 

  • If they’re on the same domain, it sends the message immediately.
  • If they’re on different domains, the MTA runs a Domain Name System (DNS) lookup to identify the receiving domain’s MX records.
  • If an SMTP server is available for the target domain, the server initiates a connection and sends the message to the intended destination.

Strengths of an SMTP Server

  • They process messages very quickly. The servers communicate directly with each other, so the message goes from one point to another without hiccups.
  • They are reliable and redundant. If it attempts to send a message but isn’t successful, it’ll re-attempt to send the email as often as possible until it goes through.
  • Organizations can set up their own servers to handle sensitive messages in-house which doesn’t cost them much.
  • They can process a large volume of emails simultaneously.

Types of SMTP Servers

Regular Server 

This type supports both inbound and outbound email deliveries and is responsible for acknowledging and queueing emails to be delivered. Afterward, it identifies the destination server of the message and sends it there, just as we described above. 

These servers may belong to email providers like Gmail and Outlook or organizations that have opted to host their own servers for security and privacy purposes.

Proxy Server

This type is a special type of server that connects to a regular SMTP server. It only works for inbound emails and is often used as a supplementary to provide for more flexible email routing. It can also monitor messages and block malicious content according to specified rules.


SMTP servers enable the exchange of billions of emails each day. We’ve explained how it works and the benefits of using it. To fully employ the potential of email technology, you can choose from one of many SMTP providers, like UniOne, which lets you send large volumes of email with its easy and affordable SMTP API.

Related Articles

Leave a Reply

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

Back to top button