What is SMTP (Simple Mail Transfer Protocol)

SMTP is the reason email systems can understand each other.

An email provider, a website, and a company’s mail server can all use different technologies. Still, they need a common way to communicate when an email has to move from one system to another. SMTP, or Simple Mail Transfer Protocol, provides those rules.

So, what is SMTP? It is the standard protocol used to send and relay email messages. SMTP helps an email client submit an outgoing message to a mail server and helps mail servers transfer messages toward the recipient’s mail system.

But SMTP does more than simply move a message from point A to point B. It works alongside DNS, email servers, authentication, and modern security standards to support reliable email delivery. This guide explains what Simple Mail Transfer Protocol is, how SMTP works, what an SMTP server does, and where SMTP fits into the modern email ecosystem.

What is SMTP?

SMTP is a set of rules that email systems use to send outgoing email messages.

In simple words, SMTP works like a delivery service for email; when you send an email, SMTP helps move the message from your email application to a mail server. If the recipient uses a different email provider or domain, SMTP also helps transfer the message between mail servers until it reaches the recipient’s mail system.

For example, when you send an email from one person at example.com to another person at example.net, SMTP helps the sending mail server communicate with the receiving mail server.

SMTP mainly handles the sending and relaying of emails. Other protocols, such as IMAP and POP3, are generally used to help recipients access emails after they reach the receiving mail server.

Here’s what its full form stands for:

• Simple means the protocol was designed around a relatively straightforward message-transfer process.
• Mail refers to electronic messages, or email.
• Transfer describes moving email messages between systems.
• Protocol means a defined set of communication rules that different systems follow.

SMTP was developed so that different email systems could communicate using a common standard. This allows a message sent through one email provider to be delivered to a recipient using another provider.

How Does SMTP Work?

SMTP works through a conversation between different email systems. A simplified SMTP journey looks like this:

Sender’s email client → Sending SMTP server → Recipient’s mail server → Recipient’s inbox

The actual process can involve more than one server, depending on the sending and receiving setup. It can also include authentication checks, DNS lookups, and security checks before the email reaches the recipient’s mailbox. Here’s how the process works step by step.

Step 1: The Sender Creates an Email

The process starts when someone writes an email using an email application or service. The message includes the recipient’s address, subject line, email content, and any attachments. The sender may also add CC or BCC recipients. Once they click Send, the email application prepares the message and starts the sending process. However, the email has not reached the recipient’s mail server yet.

Step 2: The Email Connects to an SMTP Server

The email application sends the outgoing message to an SMTP server. This server may belong to the sender’s email provider, company, or a third-party email service. The server may require authentication to confirm that the user or application is allowed to send email through it. Once the SMTP server accepts the message, it checks the recipient’s email address to find out where the message needs to go.

Step 3: The SMTP Server Finds the Recipient’s Mail Server

The SMTP server looks at the domain in the recipient’s email address, which is the part after the @ symbol. For example, in [email protected], the domain is example.net. The SMTP server then performs a DNS lookup and checks the domain’s MX (Mail Exchange) records. These records tell the sending server which mail server is responsible for receiving email for that domain.

Step 4: The SMTP Server Connects to the Receiving Mail Server

After finding the destination, the sending SMTP server connects to the recipient’s mail server. The two servers communicate using SMTP commands and responses to transfer the message. Before accepting it, the receiving server may check the sender, recipient, sending IP address, and other security signals. It may also review the email for spam or abuse. If the server accepts the message, it takes responsibility for delivering it further.

Step 5: The Receiving Server Processes and Stores the Email

After accepting the message, the receiving mail server processes it and prepares it for the recipient’s mailbox. It may run spam and malware checks and review SPF, DKIM, and DMARC results. These checks help the server decide whether the email should reach the inbox, go to spam, be quarantined, or be rejected. If the message passes these checks, the mail server stores it in the recipient’s mailbox.

Step 6: The Recipient Accesses the Email

The recipient can now open their email application and view the message. Depending on the email setup, protocols such as IMAP or POP3 may help the email application access messages stored on the mail server. SMTP’s main job is to send and transfer email. Once the receiving server accepts and stores the message, other systems help the recipient retrieve, read, and manage it.

SMTP and Email Authentication

SMTP authentication and email authentication are both related to email security, but they do different things. SMTP authentication checks if a user or application is allowed to send email through a specific SMTP server. Email authentication checks whether an email can be trusted as it moves between different email systems.

Modern email authentication mainly uses three standards: SPF, DKIM, and DMARC. Receiving mail servers use these standards to check whether an email is allowed to use the sender’s domain and whether important parts of the message look legitimate.

These standards do not replace SMTP. Instead, they add security checks to the email delivery process. This is important because SMTP alone was not designed to stop someone from using another domain in the visible From address.

SMTP and SPF

SPF, or Sender Policy Framework, allows a domain owner to publish a list of servers and IP addresses that are allowed to send email for that domain. When a receiving mail server gets an email, it can compare the server that sent the message with the sending sources listed in the domain’s SPF record. If the sending server is allowed, the SPF check can pass. If it is not on the list, the check can fail.

However, SPF has one limitation: it checks a domain used during the SMTP sending process, often called the envelope sender domain. This domain may not always be the same as the domain a recipient sees in the visible From address.

SMTP and DKIM

DKIM, or DomainKeys Identified Mail, adds something called a DKIM signature to an outgoing email. The receiving mail server can check this signature using a public key published in the sender’s DNS records.

A valid DKIM signature helps show that the email was signed by an authorized domain and that the signed parts of the message were not changed during delivery. This is useful because an email can pass through several systems before it reaches its final destination.

DKIM is not only about showing where an email came from. It also helps receiving mail servers check whether important parts of the message stayed unchanged after it was originally signed.

SMTP and DMARC

DMARC, or Domain-based Message Authentication, Reporting, and Conformance, builds on SPF and DKIM. It adds an important requirement called alignment, which checks whether the domain authenticated by SPF or DKIM matches the domain shown in the email’s visible From address.

A DMARC policy also tells receiving mail servers how the domain owner wants them to handle emails that fail DMARC checks. Depending on the policy, the email may be monitored, sent to spam, or rejected. DMARC also gives domain owners reports about emails using their domain. Analyzing DMARC reports can help them find legitimate sending services they may have forgotten about and spot possible domain spoofing or unauthorized email activity.

Together, SPF, DKIM, and DMARC give receiving mail servers more information to help them decide whether an email claiming to come from a domain should be trusted.

How Does SMTP Handle Failed Emails?

SMTP handles email delivery problems in different ways. The receiving server sends response codes to indicate to the sending server whether it accepted, rejected, or could not receive the message. The following are the two main types of SMTP errors:

Temporary SMTP Errors

A temporary SMTP error means the email cannot be delivered right now, but the problem may be fixed later. For example, the receiving mail server may be temporarily unavailable, too busy, or facing a network problem.

In this case, the sending SMTP server usually keeps the email in a mail queue instead of deleting it. It then tries to deliver the message again after some time. The server may continue trying for a set period, depending on its settings. If the problem is fixed, the email can still be delivered successfully. However, if the server keeps rejecting the message or the retry period ends, the email may fail and bounce back to the sender.

Permanent SMTP Errors

A permanent SMTP error means the email is unlikely to be delivered, and trying again usually will not fix the problem.

Common reasons include:

• The recipient’s email address does not exist.
• The recipient’s domain is invalid or cannot receive emails.
• The receiving server permanently rejects the message.
• The message is blocked because of a security or policy rule.

In these cases, the sending server usually stops trying to deliver the email. The sender may receive a bounce message, also called a non-delivery report, explaining why the email could not be delivered. The error details can help the sender or email administrator understand what went wrong.

How to Set Up an SMTP Server

The exact steps for setting up an SMTP server depend on whether you manage your own mail server or use an email provider or SMTP service. In most cases, you need to choose how you want to send emails, connect your application or email client to the SMTP server, secure the connection, and set up the right DNS records.

The following are the basic steps for setting up and configuring an SMTP server.

Choose an SMTP Solution

First, decide how you want to send your emails. You have three main options:

• Set up and manage your own SMTP server.
• Use an SMTP server provided by your email hosting provider.
• Use a third-party SMTP service.

Running your own server gives you more control, but you are also responsible for keeping it secure, maintaining it, managing its IP reputation, preventing spam, and handling other technical work. For most businesses, using an existing email provider or third-party SMTP service is easier because the provider handles most of the technical infrastructure.

Your choice should also depend on the type and number of emails you send. For example, an application that sends password reset and order confirmation emails may need a different setup than a business that mainly sends emails between employees.

Configure the SMTP Host and Connection Settings

Your email application or client needs the correct details to connect to the SMTP server. One of the most important settings is the SMTP hostname, which may look like this: smtp.example.com

You will also need the correct port and security settings. Your SMTP provider usually gives you these details when you set up the account or service. Also, make sure the hostname is correct before moving forward. Even a small mistake in the server address can stop the application from connecting and prevent your emails from being sent.

Select the Correct SMTP Port

SMTP uses different ports depending on how the connection is configured. For modern email sending, port 587 is commonly used with STARTTLS, while port 465 is commonly used for SMTP connections that use TLS from the start. The right port depends on your provider and setup. You should always use the port recommended by your SMTP service instead of choosing one yourself.

Your network or hosting provider may also block some SMTP ports. If your application cannot connect to the SMTP server even though the settings are correct, check whether a firewall, hosting provider, or network rule is blocking the connection.

Configure SMTP Authentication

Most SMTP servers require authentication before they allow an email application or service to send messages. This helps stop unauthorized people from using the server to send spam or other harmful emails.

Depending on the provider, you may need to use:

• A username and password
• OAuth authentication
• An application password
• An API-based or another supported authentication method

Use the exact authentication method recommended by your provider. For example, some providers no longer allow normal account passwords for SMTP connections. Instead, they may require OAuth or application-specific passwords.

Keep your SMTP login details safe, especially when they are used by websites or applications. Avoid putting passwords directly in public code or configuration files that other people can access.

Enable TLS Encryption

Your SMTP connection should use TLS encryption. TLS helps protect the connection between your email application and the SMTP server so that the information being sent cannot be easily read or changed while it moves across the network.

The type of encryption depends on the port and your provider’s settings. Some setups use STARTTLS, which starts with a normal connection and then makes it encrypted. Others start using TLS from the beginning. Additionally, ensure your encryption setting matches the SMTP port you are using. For example, using the wrong port and encryption settings together can cause connection errors, even when your SMTP hostname and login details are correct.

Configure Your DNS Records

If you send email from your own domain, setting up your DNS records is an important part of the process. Your email service may give you specific DNS records to add, and you should copy them exactly as provided.

Depending on your setup, you may need to configure:

• MX records to tell other servers where your domain receives email.
• SPF records to show which services are allowed to send email for your domain.
• DKIM records to publish the public key used to check your email signatures.
• DMARC records to tell receiving servers how to handle emails that fail authentication.

Check these records carefully before publishing or changing them. DNS mistakes can cause email authentication failures and delivery problems, or stop legitimate emails from reaching recipients.

You can use EasyDMARC’s DNS Record Lookup to check MX and other DNS records and make sure they are published correctly. EasyDMARC’s SPF Record Checker can help you find SPF problems, including setup errors and the 10-DNS-lookup limit. EasyDMARC’s DKIM Record Checker can help you check whether your DKIM record and public key are correctly published.

Test the SMTP Setup Before Sending Real Emails

After setting up your SMTP server, send a test email to make sure your application can connect to the server and authenticate successfully. Check whether the email reaches the receiving server and arrives where you expect it to.

A successful test should check more than whether the email was sent. Look at the email headers and authentication results to see if SPF and DKIM passed. You should also check whether the visible From domain matches your authentication setup and whether your DMARC record is working as expected.

This final check is important because an SMTP server can send an email successfully even when your DNS or email authentication setup has problems. Finding these issues before sending important emails can help you avoid delivery failures and emails landing in the spam folder.

Beyond SMTP: Building a Secure Email Setup 

SMTP has been an important part of email for many years. Its main job is simple: helping emails move from one system to another. Every time you click Send, an SMTP server can help start your email’s journey.

However, sending an email is only one part of the process. Good email delivery also depends on the right SMTP settings, correct DNS records, email authentication, and a good sender reputation. Understanding how SMTP works can help you find and fix email delivery problems.

As email threats continue to grow, businesses need to do more than just send emails. They also need to protect their domains with SPF, DKIM, and DMARC.

With EasyDMARC, you can manage your email authentication setup from one platform and get clear insights into your domain’s email security. Start your 14-day free trial today and see how EasyDMARC can help you better protect your domain.

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