SMTP: Everything You Must Know

SMTP.js: Everything You Must Know

Email is an essential communication tool for personal and business use. But what makes it work? The answer lies in SMTP – the Simple Mail Transfer Protocol.

As a beginner, it’s important to understand what SMTP is, how it works, why it’s needed, and how to use it properly. This comprehensive guide covers all the SMTP basics and more to give you a solid foundation.

What is SMTP?

SMTP stands for Simple Mail Transfer Protocol. It’s an application layer protocol that transfers email reliably and efficiently between email servers over the internet.

In simpler terms, SMTP allows you to send emails from your device to another person’s email inbox and vice versa. It defines the message format, communication rules, commands, and responses needed to transfer your email data from point A to point B.

Without SMTP, email communication as we know it would not be possible.

Sending Emails Made Easy Integrating Nodemailer with Reactjs

Integrate your SMTP to react apps. Check this article – Sending Emails Made Easy Integrating Nodemailer with Reactjs.

How Does SMTP Work?

Behind the scenes, this is what happens when you send an email:

  1. Your email client (e.g., Gmail, Outlook) connects to your local SMTP server and submits your email message data using SMTP commands.
  2. Your local SMTP server adds extra information, like timestamps, message boundaries, etc., to format the message according to SMTP standards.
  3. Your local SMTP server looks up the recipient’s email domain via DNS to find out the mail server responsible for that domain.
  4. Your SMTP server opens a connection with the recipient mail server and transfers your email using more SMTP commands.
  5. The receiving SMTP server receives and processes the incoming email before depositing it in the recipient’s mail inbox.
  6. The recipient can now access your email using protocols like IMAP or POP3 when they check their email inbox.

As you can see, SMTP handles the sending, transit, and routing of your email between servers. Without it, your email would never leave your outbox, let alone reach its intended destination.

Why is SMTP Needed?

There are a few reasons why SMTP is a necessity for email to work:
Reliability – SMTP establishes reliable connections, ensures error-free transmission with delivery status notifications, and handles retries if delivery fails initially. This minimizes email loss/corruption.

Interoperability – SMTP can transfer emails between varied systems despite differences in underlying network architecture. This enables global email communication.

Scalability – SMTP can handle huge volumes of email traffic efficiently. Peak email volume exceeds 300 billion emails daily.

Security – SMTP supports security extensions like SMTPS (SMTP over SSL/TLS) to encrypt connections, preventing email hijacking or theft.

Speed – SMTP is optimized for fast email relaying and transfers, enabling near real-time communication.

In essence, SMTP provides a standardized framework so mail servers and email clients can communicate despite their technical differences.

Key SMTP Components

To understand SMTP better, you need to become familiar with its main components:
Mail User Agent (MUA) – The email client software (e.g., Gmail, Outlook, Apple Mail) used to read/compose emails.

Mail Submission Agent (MSA) – Accepts outbound emails from clients and submits them to the Mail Transfer Agent using SMTP.

Mail Transfer Agent (MTA) – Server software relays emails using SMTP. Common MTAs include Postfix, Sendmail, and Exchange Server.

Mail Delivery Agent (MDA) – Server software that delivers incoming emails to recipient inboxes. Often the same as MTA.

Simple Mail Transfer Protocol (SMTP) – Application layer protocol governing email data exchange between mail agents.

SMTP Commands – Standard commands like EHLO, MAIL FROM, RCPT TO, DATA, etc. used to initiate/exchange emails.

SMTP Port 25 – Standard port that SMTP uses for connection. Wrapped by delivery ports 465 (SMTPS) or 587 (submission).

SMTP Response Codes – 3-digit codes like 220, 250, 452, etc., indicating server response status after each command.

Now that you know the key pieces, it’s easier to see how they work together to send emails from start to end.

Setting Up an SMTP Server

You must set up and maintain your own SMTP server to send emails independently without relying on an ESP. Here are the basic steps:
1. Install SMTP Daemon
Choose and install suitable SMTP daemon software on your server, like PostfixSendmail, etc. This becomes your MTA.
2. Configure DNS Records
Add valid DNS A, MX records pointing to your server so emails to your domain are routed to your SMTP server.
3. Adjust Firewall Settings
Adjust the firewall to open SMTP ports for inbound/outbound email traffic. Permit port 25 for SMTP and consider 465/587 for SMTPS.
4. Create Admin & User Email Accounts
Add email accounts with mailboxes on your SMTP server that can send/receive external emails
5. Enable Authentication
Require SMTP AUTH on port 587 using usernames/passwords to secure email submission
6. Set Up Spam Filters
Implement anti-spam measures like SPF, DKIM, etc. to improve deliverability and avoid blacklists
7. Monitor Activity
Use server logs, cPanel, or tools to monitor SMTP activity, email traffic stats, performance, etc.

With the basics covered, you can now focus on optimizing, scaling, and hardening your email server for best performance.

Key SMTP Commands

SMTP commands are text instructions the sending server gives to interact with the receiving SMTP server when transferring an email.

Here are some of the common SMTP commands and responses you should know:

  • HELO

The server introduces itself and initiates the SMTP conversation
Response Code: 250

  • EHLO

Enhanced version of HELO identifying server capability
Response code: 250


Establish encrypted SMTP sessions using TLS
Response code: 220


Authenticate client with Base64 encoded login credentials
Response code: 334


Indicates sender’s email address
Response code: 250

  • RCPT TO:

Indicates recipient’s email address
Response code: 250

  • DATA

Initiate transfer of email content
Response code: 354

  • Content Data

Email headers and body sent line by line
Response Code: 250

  • QUIT

Disconnect SMTP session

You can easily debug SMTP connection issues by becoming familiar with these basic SMTP commands.

How to Test an SMTP Server

It helps to manually test your SMTP server using Telnet or an SMTP diagnostics tool when setting up or troubleshooting issues.

Here is how you can test basic SMTP connectivity from the command line:

  1. Open the Command Prompt or Terminal window
  2. Type: telnet [YOUR-SMTP-SERVER] 25
  3. Check the 220 response code after connecting
  4. Type SMTP commands and check the responses
  5. Type QUIT when done

Some key things to validate – the SMTP server responds on connect, default ports are accessible, commands work as expected, and authentication and TLS encryption are enforced.

This quick sanity check can reveal potential misconfigurations or blockers preventing your SMTP server from working properly for critical functions like bulk email delivery.

Securing Your SMTP Server

Since your SMTP server directly connects to external servers for email transfers, securing it against exploitation is paramount.

Here are some best practices to lock down your SMTP environment:

  • Enforce TLS encryption for secure data transfer
  • Enable SMTP authentication to validate senders
  • Install spam filters like SpamAssassin
  • Create allow/block lists to filter mail relay access
  • Set up SMTP connection limits per client
  • Log and monitor all SMTP activity closely
  • Use DMARC, SPF, DKIM to prevent spoofing
  • Disable open relaying and restrict mail forwarding
  • Isolate SMTP server from public Internet access
  • Keep the SMTP software constantly updated
  • Harden the OS and close unused ports
  • Setup monitoring and alerting for anomalies

Additionally, periodically conduct external and internal penetration tests to uncover and fix security holes before attackers do.

Following strong SMTP hardening measures greatly minimizes the risks and prevents your infrastructure from inadvertently becoming an engine for spreading spam, malware, or phishing scams at scale.

Using APIs to Send Email

Setting up your own SMTP servers takes considerable effort. For many, using third-party email services like SendGrid, Mailgun, etc., is more practical.

These email service providers abstract away the complexity of email infrastructure and provide user-friendly APIs to send emails from your web apps directly.

For example, to send email using SendGrid’s API with Node.js:

// Using SendGrid's v3 Node.js Library
const sgMail = require('@sendgrid/mail') 


const msg = {
  to: '',
  from: '',
  subject: 'Hello from SendGrid',
  text: 'This email was sent using SendGrid via API'

  .then(() => {
    console.log('Email sent successfully');
  .catch((error) => {

Here we are:

  • Importing the SendGrid mail library
  • Supplying our API key
  • Composing the email with to, from, and subject fields
  • Sending off the email
  • Logging the response

And that’s all there is to it! Behind the scenes, SendGrid connects to SMTP servers securely and gets the email delivered quickly and reliably.

Most email service providers will have similar APIs to simplify email sending in languages like Python, PHP, Ruby, C#, Java, etc.

Benefits of using a mail API:

  • No need to run your own SMTP servers
  • Simple integration with few lines of code
  • Guaranteed email deliverability
  • Built-in analytics and insights
  • Scales to any email volume easily
  • Provides other advanced email features

So, unless you plan to send many emails, using a reliable third-party delivery API is the easiest way to add email capabilities to your apps.

Frequently Asked Questions

What port does SMTP use?

The default SMTP port is 25. But for encrypted connections, ports 465 and 587 are often used.

Is SMTP faster than POP3 or IMAP?

A: Yes, SMTP is optimized for faster email relaying between mail servers compared to protocols used for mailbox access.

Can I send emails without an SMTP server?

A: No, you must have access to an SMTP mail server either by running your own or using a third-party email delivery service to be able to send emails.

What’s the difference between SMTP and API Email?

A: SMTP requires you to install, configure, and manage mail servers, whereas API Email allows sending emails easily via a third-party service using their web API without running SMTP servers.

Is setting up an SMTP server difficult?

A: Configuring your performant and secure SMTP mail server requires in-depth technical expertise in email infrastructure, networking, server security, DNS management, etc. Using a third-party email sending API service is easier.

What’s the latest version of SMTP?

A: The current standard is defined in RFC 5321 which obsolete the older RFC 821 standard published in 1982. Enhanced extensions to SMTP are defined in RFC 1869.

Grasping these SMTP basics is vital for anyone planning to send professional emails at scale, whether for transactional needs, newsletters, or other bulk requirements down the line.


We have covered a lot of ground explaining the key aspects of SMTP servers for beginners. Let’s recap the main points:

What is SMTP? – SMTP or Simple Mail Transfer Protocol is the standard method for sending emails reliably over the internet. It defines a common language for communication between different mail servers.

Why is it Needed? – SMTP enables the seamless transfer of emails globally across different systems. It provides reliability, security, speed, and interoperability.

How Does it Work? – Your Mail User Agent uses SMTP commands to submit your email from your device to your mail server. Your SMTP mail server then relays it to the receiving server using SMTP commands, which deposit it in the recipient’s inbox.

Components – Key components that work together to make SMTP email transfer possible include the clients, servers, protocols, ports, and commands.

Setting Up – To configure your own SMTP server, steps like installing a mail transfer agent, configuring DNS records, adjusting firewall settings, creating email accounts, etc., are required.

Security – Encryption, authentication, filters, access controls, and strict hardening are a must to lock down an SMTP server securely.

APIs – Third-party services like SendGrid provide simple APIs to send email without running your own SMTP server.

We also extensively covered SMTP ports, commands, testing procedures, and comparisons between protocols like SMTP, POP3, and IMAP.

With this knowledge, you now have a solid basis to work with SMTP servers whether you want to set up your own or use a third-party service for your email needs.


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