What is Simple Mail Transfer Protocol (SMTP)?

New communication innovations may come and go, but email remains a constant, both then and now. People all around the world rely on emails for business and professional communication, sending messages in just a click. But have you ever wondered how emails travel across cities and countries in seconds?

If so, it's time to meet SMTP (Simple Mail Transfer Protocol)—the behind-the-scenes hero ensuring your emails move seamlessly from one server to another, anytime, anywhere.

Diving into SMTP can feel like exploring an ocean, so we’ve broken it down into easy-to-understand sections for readers of all levels. Let’s get started!

What is SMTP?

Simple Mail Transfer Protocol (SMTP) is an email protocol used for email sending from one email client to another client or the same client via the Internet. It's nothing but a set of rules that initiates email sending, receiving, and relaying.  

The role of SMTP

The role of SMTP is pretty simple. It acts as a bridge between senders and receivers, ensuring they "speak" the same language. With simple rules and easy-to-follow commands, SMTP keeps emails moving without interruption. 

What's the difference between SMTP and SMTP relay?

We often tend to confuse these two terms while understanding the concept of SMTP. SMTP is the protocol, while SMTP relay is the process of forwarding emails between servers to ensure reliable email delivery.

• SMTP is simply a protocol. It sets the rules that ensure email servers communicate in the same language for smooth email delivery.
• SMTP relay functions like a real-world relay, where servers pass emails from one to another until they reach the recipient.
   

Why is SMTP crucial in transactional email sending?

1. Creates secure communication

SMTP supports encrypted transmission using TLS (Transport Layer Security) and SSL (Secure Sockets Layer) to protect sensitive information. Secure SMTP connections prevent unauthorized access and ensure that emails are safely delivered without being intercepted.

2. Authentication for reliable email delivery

To prevent email spoofing and unauthorized access, SMTP requires authentication before processing email requests. This ensures that only verified users and servers can send emails, improving security and reducing spam risks.

3. Simple, yet effective protocol

Unlike complex communication protocols, SMTP operates using a set of easy-to-understand commands, making email transmission straightforward. It efficiently handles sending, relaying, and forwarding emails.

What role does SMTP play in sending emails through Zoho ZeptoMail?

Now that you know what SMTP is, let's see how a SMTP relay service like Zoho ZeptoMail uses SMTP for message submission and relaying. 

1. Secure communication between the client and ZeptoMail

When a client sends an email through ZeptoMail, it must establish a secure and authenticated connection to prevent unauthorized access and ensure data integrity. With the help of SMTP, a secure connection is established for message submission.

A. ZeptoMail only accepts email submissions over secure ports:

• Port 465: Uses SSL (Secure Sockets Layer) for encrypted communication.
• Port 587: Uses TLS (Transport Layer Security) to ensure secure email transmission

B. Authentication requirement (AUTH command)

• Before sending emails through ZeptoMail’s SMTP, clients must authenticate using the AUTH command.
• Authentication will be done only from the IPs you add in your restricted IP list.
• This ensures that only authorized users can send emails, preventing spam and phishing attempts.
   

2. Secure communication between ZeptoMail and the recipient

Once ZeptoMail receives the email, it ensures secure delivery to the recipient’s server by SMTP relaying. SMTP helps with simple communication and tries to speak the language which the recipient server is comfortable with.

TLS/opportunistic TLS for encryption:

• ZeptoMail attempts to establish a TLS-encrypted connection with the recipient’s email server whenever possible.
• If the recipient server supports TLS, the email is transmitted securely, ensuring data protection.
• If TLS is not supported, ZeptoMail establishes an opportunistic TLS connection to transmit the messages securely to the recipient.
   

How do you configure SMTP to send transactional emails via ZeptoMail?

Here's a quick roadmap of how to configure your SMTP with ZeptoMail.

Step 1: Create a Mail Agent

• In ZeptoMail, create a Mail Agent, which acts as a container to manage your email settings and analytics.
• Each Mail Agent will have its own SMTP credentials, including an SMTP username (API key) and password.
   

Step 2: Obtain SMTP credentials

• Go to the Mail Agent you created in the ZeptoMail console.
• Under the "SMTP" section, you'll find the SMTP server details, including the SMTP host, port, and credentials (API key as the SMTP username and a password).
   

Step 3: Configure your application

Use the SMTP credentials from ZeptoMail to configure your email-sending application. Here's an example setup:

SMTP server settings:

• SMTP Host: smtp.zeptomail.com
• SMTP Port: 465 (SSL) or 587 (TLS)
• SMTP Username: Email API key (provided by ZeptoMail)
• SMTP Password: The password you generated in the ZeptoMail console

Note: In order to eliminate the security vulnerabilities of the older TLS versions, ZeptoMail supports only TLSv1.2 for SMTP.

To get a detailed understanding of the process, read our help article on SMTP configuration.

What are the different SMTP error codes in ZeptoMail?

SMTP error codes are response codes that SMTP servers send to indicate what happened to messages. They're similar to road signs, with green lights indicating success, yellow lights indicating temporary issues, and red lights indicating permanent problems.

Wrapping up

By now, you’ve grasped what Simple Mail Transfer Protocol (SMTP) is and how ZeptoMail stands out as a transactional email service. ZeptoMail focuses exclusively on sending secure and timely transactional emails via SMTP, ensuring reliable delivery. Understanding SMTP is crucial for seamless email communication, and ZeptoMail leverages it to deliver your emails efficiently.