Last updated
November 17, 2023

Understanding SMTP and Its Essential Components

Brian Wu

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Have you ever wondered how your email magically reaches its intended recipient across different parts of the world within seconds? 

Behind the scenes, a carefully orchestrated process ensures that your emails are routed to their destinations accurately, swiftly, and securely. This fundamental mechanism of email communication relies on a protocol that has been around since the 1980s and is indispensable even today. 

In this guide, we’ll look at the secret behind this technology, how it works, why it is important, and explore a few ways you can leverage SMTP to enhance your email deliverability

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What is an SMTP Server?

SMTP, or Simple Mail Transfer Protocol, is the standard protocol for sending email messages across the Internet. It originated in the early 1980s, with RFC 821 being its first documented version. 

Over the decades, SMTP has undergone several revisions, yet its primary function remains unchanged: to ensure the delivery of email messages to their intended recipients. SMTP operates silently in the background of email communication, handling the flow of messages from the sender to the recipient's email server.

The Role of an SMTP Server vs. Email Server

While you might see mail server and SMTP server used interchangeably, this is not always correct. An SMTP server actually has a few notable differences from a regular mail server. 

While an email server stores and manages email messages, an SMTP server specifically deals with the sending and routing of these messages. It follows the rules laid out by the SMTP protocol to relay emails from the sender's email client to the recipient's email server, acting as an intermediary that ensures emails are returned to sender if they don’t go through.

How SMTP Servers Work

So, we know that we need SMTP for email to work. But how does it work? We can compare SMTP to how a well-coordinated passenger train system works, ferrying passengers—or emails—quickly and efficiently.

When you send an email, your email client acts as the departure station, while the SMTP server acts as the train that will transport your email to its destination. Let’s break it down step by step.

1. Establishing the Connection

Initially, a connection is established between your email client (SMTP client) and the SMTP server via a Transmission Control Protocol (TCP) connection, akin to a train ready to depart from the station. This connection is made over specific pathways, or ports, with port 25 or 587 being the common ones used for SMTP, just as trains depart from specific platforms.

You might also see this referred to as the Mail User Agent (MUA). Either way, this is your email client, such as Gmail or Outlook, that you use to send and receive email messages. When you hit the 'send' button on your email, the MUA connects to the SMTP server to initiate the email-sending process.

2. The Digital Handshake

Once your email client connects to the SMTP server, it's like having your email boarded onto the train. At this stage, a digital handshake occurs where the SMTP client sends a 'HELO' or 'EHLO' command to the SMTP server, identifying itself and initiating the SMTP session. This step is analogous to checking the ticket and schedule before the train departs. 

3. Verification and Routing

The SMTP server then checks the credentials (authentication) and the recipient’s address verification to ensure the email can be properly routed, similar to checking the destination tag on a piece of luggage.

4. Delivery Process

Now, the SMTP server, acting as a Mail Transfer Agent (MTA), takes on the task of delivering your email. It consults the digital directory, the Domain Name System (DNS), to find out the IP address of the recipient’s SMTP server, much like how a train system refers to a timetable to find out which station the train needs to go to next.

5. The Email Journey

The journey begins. The SMTP server relays your email over the internet to the recipient’s SMTP server, akin to the train traveling towards the destination station. This relay occurs through a series of TCP/IP protocols, ensuring the email data packets are correctly sequenced and error-free, just as a train follows a predetermined track and schedule.

6. Arrival Confirmation

Upon reaching the recipient’s SMTP server, a confirmation is sent back to the original SMTP server to acknowledge that the email has arrived safely, similar to how a train’s arrival is confirmed at the destination station. This acknowledgment typically comes in the form of a 250 OK status code, signaling the successful delivery of the email.

7. Completion of Delivery

After this acknowledgment, the original SMTP server knows its job is done, and your email rests in the recipient’s inbox, ready to be read, much like how a passenger disembarks at the destination station with the assurance that they have arrived safely. 

Discovering Your SMTP Email Server Address

The SMTP server address is the network address of the SMTP server that your email client uses to send email messages. Different email services or providers have their own SMTP server addresses. You can usually find this in your email provider’s account settings or developer documentation.

For instance, the SMTP server address for Gmail is, and for Outlook, it's 

SMTP Email and Common SMTP Commands

SMTP influences email deliverability, preventing spam and ensuring the security of email communications. SMTP commands are directives used to communicate actions or statuses between an email client and a server during the email sending process. 

Here are some of the most common SMTP commands you’ll see.

  • HELO: Initiates the SMTP conversation with the server, identifying the client.
  • EHLO: Starts the conversation and signals the server that the client may use Extended SMTP (ESMTP) commands.
  • MAIL FROM: Specifies the sender's email address.
  • RCPT TO: Identifies the recipient's email address.
  • DATA: Indicates the beginning of the message body.
  • QUIT: Ends the SMTP session and disconnects.
  • RSET: Resets the current email transaction.
  • VRFY: Asks the server to confirm whether a given email address is valid.
  • NOOP: No operation; can be used to keep the connection open.

The following flowchart can help you visualize when you would use specific SMTP commands:

The Importance of Email Validation in SMTP Communication

Email validation is the process of verifying that an email address is valid and active before sending an email. It's vital because sending emails to invalid or fake addresses negatively impacts email deliverability and can tarnish your sender reputation.

Your sender reputation can be thought of as a credit score for your email sender identity. Consistently delivering emails to invalid or non-existent addresses leaves a negative mark on your reputation. A good sender reputation is key if you want people to receive and open your emails.

You can think of email validation as a gatekeeper that only permits legitimate email addresses through. It's like cross-checking a shipping address against a navigation system—if the address doesn't exist, the package cannot be delivered. 

When you verify that each email in your list is real and active, you significantly increase the odds of your emails landing in the inbox of your recipients, and actually getting your message across rather than bouncing back or being filtered as spam.

Introducing AbstractAPI's Email Validation API

AbstractAPI's Email Validation API is a tool designed to ensure the validity of emails. It checks emails against various parameters, identifying and filtering out invalid or risky addresses. You also get handy features for identifying emails from disposable and free providers, suggesting corrections for typos, and checking if the domain is active and able to receive mail. 

Additionally, the API assesses the risk associated with an email address, helping you maintain clean email lists and improving deliverability. 

You can get started without needing to be a tech guru. As soon as you log in, you get an easy-to-use interface and can start with just two parameters—your unique API key and the email address to be validated.

As for pricing, you can pick from a variety of pricing plans depending on your goals. The Starter plan, ideal for beginners, provides 60,000 API calls per year at a rate of $8 per month. For those with more extensive validation requirements, the Standard plan increases the available API calls to 600,000 per year for $45 per month. If you have even more extensive requirements, you can jump to an even higher level—the point is, it is as flexible as you need it to be.

Common Misconceptions About SMTP

Some common misconceptions about SMTP include the idea that it's an outdated protocol, or that all email servers are SMTP servers. In reality, SMTP continues to be a reliable, widely-used protocol for email communication, and not all email servers serve the function of sending and routing emails as SMTP servers do. 

Let’s break down three common misconceptions about SMTP.

SMTP is just for emails 

One common misunderstanding is that SMTP is designed only for email communications. While SMTP is primarily used for transmitting emails, it was not created exclusively for that purpose. Originally developed decades ago as a replacement for older electronic messaging systems, SMTP has evolved to include features like authentication tokens, improving upon its performance and functionality for a variety of data transfer needs.

SMTP is just a server

There's also confusion about what an SMTP server is. It's not just a standalone server; rather, SMTP involves a client-server model where the client application on a local machine communicates with a remote device running the server application. This relationship facilitates the delivery of messages across networks, not just between individual servers

SMTP operates alone

In reality, SMTP servers often interact with various external entities to ensure email deliverability, taking into account factors like security and privacy. For instance, Gmail's SMTP process involves several verification steps before an email reaches the inbox, including checks against blacklisted domains and DNSBL servers for blacklist verification​


Understanding SMTP and its role in email communication is crucial for both businesses and individuals. It's the backbone of email communication, and having a solid grasp of how it works can help you improve your email deliverability and security. 

For further reading or more tech insights, subscribe to our newsletter or visit our blog! Now lets answer some commonly asked questions about SMTP.

Frequently Asked Questions

Is an SMTP address the same as an email address?

No, an SMTP address is not the same as an email address. An SMTP server address is a specific address that mail clients use to send emails via SMTP. It typically looks like "". For example, Gmail's SMTP server is "".

What is the port for SMTP?

SMTP traditionally uses TCP port 25 for communication between mail servers. However, mail clients generally use specific "submission" ports for sending emails—typically port 587—the standard port for email submission over TLS.

How does message transfer work in SMTP?

Message transfer in SMTP involves a series of commands and responses that occur between an email client and the server. After the initial HELO (or EHLO for Extended SMTP) command, the client sends the MAIL FROM command to specify the sender's email address, followed by RCPT TO to specify the recipient's address. Then, the DATA command begins the transfer of the email content. Once the message is completed, the QUIT command ends the SMTP conversation.

What is an SMTP service?

An SMTP service is a third-party platform or server that handles the sending, receiving, and routing of email messages on behalf of clients. It operates using the SMTP protocol to ensure proper communication between email servers and to facilitate reliable email deliverability. These services often provide additional features such as email tracking, analytics, and security measures.

Do I need an SMTP server to send email?

Yes, you need an SMTP server to send email. The SMTP server acts as a post office that processes your email, decides which server to send the message to, and then relays the message to that server. Without an SMTP server, your email won't be able to reach its destination, as it is the technology behind email communication that moves messages from one point to another​.

What is the difference between SMTP and POP3?

SMTP and POP3 (Post Office Protocol version 3) serve different functions in email communication. SMTP is a protocol used for sending emails from the sender’s system to the recipient's mail server. It operates as a message transfer agent and follows a process that involves the sender's mail server and the recipient's mail server​​. POP3, on the other hand, is used for receiving emails. It acts as a message access agent, allowing the retrieval and organization of emails from the recipient’s mail server to the recipient’s personal computer. POP3 involves a client and server message access agent, where the client-side software pulls the email from the server to the local machine, typically using TCP port 110.

What is the difference between SMTP and IMAP?

SMTP and IMAP (Internet Message Access Protocol) are two distinct protocols used for different aspects of email handling. While SMTP is used for sending, relaying, or forwarding messages from a mail client to a receiving email server, IMAP manages and retrieves email messages from the receiving server. Unlike SMTP, IMAP is used for receiving messages, not sending them. After the SMTP transmission is complete, the recipient's email client will fetch the message using IMAP, which allows the email to be synchronized and accessed across multiple devices.

What role does TCP/IP play in SMTP email delivery?

TCP/IP, or Transmission Control Protocol/Internet Protocol, is crucial in SMTP email delivery as it establishes the connection between the client and the SMTP server during an SMTP session. This TCP connection starts with a handshake and is used to transfer email messages reliably across the network.

Brian Wu
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