What is SMTP and How Does it Work?

What is SMTP, and why is it essential for reliable email delivery?

SMTP, or Simple Mail Transfer Protocol, serves as a set of rules for how emails should be sent and received over the internet. Think of it as the postal service of the digital world, ensuring your emails reach their intended destinations.

SMTP works like a global shipping network for emails. It packages your message, addresses it correctly, routes it through various “distribution centers” (servers), and delivers it to the recipient’s inbox. If delivery fails, it returns the email to the sender.

Let’s break down this process a bit further. 

Connecting to the server: First, the sender’s mail server (SMTP client) initiates a Transmission control protocol (TCP) connection with the recipients SMTP server.

Using ports: SMTP uses ports to facilitate communication between email clients and receiving email servers. Think of it like different ports that ships sail into. Different ports are better for different types of data. You can select which SMTP port to use but the default is port 25.

These next steps involve SMTO commands, which are just communications and response confirmations between sending and receiving servers.

Greetings and handshake: Servers are polite. The sending server will send a “HELO” or “EHLO” command to the receiving server to identify itself. “EHLO” is a version of “HELO” where the command also declares the server’s capabilities.

Sender information: The sender’s server sends the “MAIL FROM” command with the sender’s email address, and the recipient server acknowledges receipt with a 250 status code.

Recipient information: The sender’s server sends a “RCPT TO” command with the recipient’s email address and the recipient server acknowledges with a 250 status code.

Data transfer: Now it’s time to send the actual data. The sender’s server sends the “DATA” command to start the message transmission.

Closing the connection: Once the full message has been sent, the sender’s server sends a “QUIT” command to close the connection. The recipient’s server sends back a 221 status code to confirm a successful transmission.

In other words, SMTP is the behind-the-scenes protocol that makes email communication possible.

Sorry in advance, but here’s another acronym. IMAP, or Internet Message Access Protocol, is a standard protocol used by email clients to retrieve emails from a mail server.

Why are we bringing it up? SMTP is responsible for sending outgoing emails from a client server and delivering them to the recipient’s server. So, while SMTP handles sending emails – IMAP is all about receiving them on the mailbox provider side (think Gmail and Yahoo) providing users with a seamless email experience across multiple devices.

This section is all about infrastructure. We’ve outlined a basic explanation above of how SMTP works step-by-step, but there’s more to it than just server conversation between senders and receivers.

There are three main components we need to look at.

1. SMTP server

We’ve already talked about this a bit – you just may not have realized. The SMTP server receives emails from email clients like Gmail and Yahoo and transfers them to other SMTP servers or incoming mail servers. These SMTP servers can be cloud-based (from a provider like us) or you can maintain your own on-premises solution. But we’ll get into that later.

2. SMTP relay

SMTP servers transfer messages to other SMTP servers, this is called SMTP relay. You need SMTP relay if you are transmitting messages between servers that are hosted on different domains. In other words, if you are sending messages from Outlook to Gmail. But we still need a way to verify these different domains for security.

3. Mail agents

Mail agents are responsible for managing and sending emails. They provide a user interface for interacting with email. In an SMTP relay, the mail agent will check the domains and format the message according to SMTP standards so it can route it to the recipient’s inbox. There are a few types of mail agents:

• Mail User Agent (MUA): The email client (Gmail, Apple mail) used to send and receive messages, and that provides the user interface.
• Mail Submission Agent (MSA): Receives email messages from the email client and validates headers and domain names.
• Mail Transfer Agent (MTA): Processes and transfers emails and can take the place of an MSA by performing its tasks.
• Mail Delivery Agent (MDA): The final agent before the emails reaches the recipients SMTP server for retrieval. (The IMAP process we mentioned earlier)

SMTP is a way of standardizing how email messages are transmitted. Because it’s the standard it ensures compatibility across different email services and platforms and allows other practices and protocols to rely on it to help you ensure efficient delivery. SMTP is also a way of doing some basic authentication to verify sender identity. While it’s not the only authentication, or the most effective, it does help prevent unauthorized access and reduce spam.

SMTP is basically a conversation between sending and receiving servers to set up a secure path for sending a message. Kind of like that meme where two men are yelling at each other across a field, “Hey! What’s your name?” It would be nice if SMTP were that easy, understanding it is crucial for anyone who relies on email communication, whether for personal or business use – but it can get technical.

For those who want more control over their email setup, understanding SMTP settings can be beneficial. These settings allow you to customize how your emails are sent and received. You can typically find SMTP settings in your email client’s configuration options, where you can specify the SMTP server, port, and authentication method.

While it’s possible to configure your own SMTP server (in fact, many web hosting services like WordPress will provide you with your own built-in SMTP server), this is the more challenging and expensive route. Using built-in SMTP servers comes with large overheads like maintaining your own infrastructure and security.

Using a built-in SMTP server can come with several security threats. Built-in servers may lack the advanced security features found in dedicated mail server software. Security is one of those things that needs to be updated frequently, and if this isn’t maintained by a provider your may be more vulnerable to attacks like spamming, phishing, and unauthorized access.

If not properly configured or updated, these built-in servers can become easy targets for hackers looking to gain access to sensitive data. Built-in SMTP servers also require a lot of maintenance (which is expensive) and you have to employ humans who can perform updates to things like security patches.

Okay, so we know the pitfalls of running your own SMTP server, but what are the benefits of using an SMTP service provider instead? In other words, how do you justify the cost when budget time rolls around?

Here are some benefits of using an SMTP service provider (also known as an SMTP relay service) to provide you with an SMTP server:

• You don’t have to take care of your own email sending infrastructure
• You’ll be able to send emails faster and in larger volumes
• You’ll have better email deliverability
• You’ll get better insight into your email sending practices

Let’s dig into each of these advantages below.

Imagine you are a large e-commerce company sending out thousands of order confirmations daily. When a customer completes a purchase, here’s how SMTP handles the confirmation email:

1. Your company’s server initiates an SMTP connection with your email service provider (like Sinch Mailgun)
2. SMTP authenticates your company’s domain and checks sending permissions
3. The protocol formats the order details into proper email structure
4. The message routes through multiple SMTP servers, each verifying the next destination
5. Finally, the email arrives at the customer’s email provider (like Gmail or Yahoo)

This process happens in milliseconds, but any failure point (authentication issues, rate limits, blacklisted IPs) can disrupt thousands of critical transactional emails. That’s why enterprise clients need reliable SMTP infrastructure with monitoring, authentication, and deliverability optimization.

This is perhaps one of the best parts of outsourcing your SMTP server needs: you don’t have to set up and maintain your own email sending infrastructure. This means you can kick back and let a dedicated technical support team manage your infrastructure so that you can focus on other parts of your email campaign. SMTP service providers will help maintain your IP and sender reputation to help you improve your deliverability.

You send mail faster and in larger volumes with a dedicated SMTP service provider. Most ISPs and email clients flag high traffic volumes as possible signs of spam. Still, SMTP service providers help you navigate that tricky minefield to keep your deliverability up and your IP or sender reputation intact. SMTP service providers can also help you optimize your send time so you don’t hit sending limits.

Your marketing team is probably busy creating great, non-spammy copy, and your designers are excellent at crafting beautiful HTML email templates that won’t trigger spam traps. But how can you chip in from a development perspective?

We’ve already gone over this above, but SMTP service providers, like Sinch Mailgun, focus on maintaining your email infrastructure and improving your deliverability. You don’t have to guess what works best to avoid spam traps. Your SMTP service provider supplies you with the tech support to stay out of trouble so you can focus on email sending.

Outside of understanding SMTP basics – and how SMTP can improve deliverability – youneed to know a bit about the specifics of SMTP commands.

SMTP commands are a set of instructions used in the Simple Mail Transfer Protocol(SMTP) to enable the transfer of email messages between servers. For example, the”HELO” command is used to introduce the mail client to the server. These commands play abig part in the efficiency and reliability of email delivery across the internet. SMTPcommands help establish a connection between the sender’s and recipient’s mail servers,authenticate users, specify the sender and recipients of the message, and transfer the emailcontent.

By following a standardized set of commands, the SMTP protocol ensures that emailservers can communicate effectively, handle errors, and manage the entire message transfer process, from the initial handshake to the final delivery.

By this point we hope you have a good general understanding of what SMTP is. Here are some best practices to keep in mind:

Here are the top three SMTP (Simple Mail Transfer Protocol) best practices to enhance email security and reliability:

Security: Encrypting SMTP connections with TLS (Transport Layer Security) is crucial for protecting sensitive email data during transmission. This prevents eavesdropping and man-in-the-middle attacks. Ensure that your SMTP server supports and is configured to use SSL/TLS for both incoming and outgoing mail.

Authentication: Enable SPF, DKIM, and DMARC: Remember, SMTP does a very basic job of authenticating sender identity. These email authentication standards help prevent email spoofing, phishing, and spamming.

• SPF specifies which servers are authorized to send emails on behalf of your domain.
• DKIM adds a digital signature to emails, allowing the recipient server to verify the sender’s authenticity.
• DMARC provides policies for how emails failing SPF and DKIM checks should be handled, reducing the chances of malicious emails reaching recipients.

Monitoring: Regularly monitor your SMTP logs for unusual activity, failed login attempts, or large spikes in outgoing emails, as these could indicate unauthorized access or a compromised account.

1. What is SMTP and why is it important? 

SMTP (Simple Mail Transfer Protocol) is the standard for sending emails across the internet. Think of it as a digital postal service that ensures emails are delivered to the right place.

• Enables email communication between servers.
• Provides error messages if emails can’t be delivered.
• Essential for seamless email operations.

2. How does SMTP work? 

SMTP works step-by-step to deliver your emails: 

• Connects your email client to an SMTP server. 
• Validates sender and recipient details. 
• Sends and routes the email through a network of servers. 
• Returns undelivered emails with error codes if issues arise. 

3. Why use an SMTP service provider?

SMTP providers like Sinch Mailgun handle the heavy lifting, so you don’t have to.

• Avoid managing infrastructure and security updates.
• Get better email deliverability and faster sending.
• Access tools like reputation monitoring and real-time analytics.

4. What challenges can arise without SMTP best practices?

Misconfigured SMTP settings can lead to delivery failures or spam issues.

• Missing authentication protocols like SPF, DKIM, and DMARC.
• No encryption, making emails vulnerable to attacks.
• Lack of monitoring, which can miss deliverability problems.

5. How does Sinch Mailgun optimize SMTP for businesses?

Sinch Mailgun simplifies SMTP email sending while boosting performance.

• High-volume email support with optimized delivery.
• Advanced tracking and analytics for transparency.
• Flexible integration with APIs and tools tailored to your needs.

When it comes to email communication, SMTP (Simple Mail Transfer Protocol) is the backbone of sending emails across networks. However, SMTP alone doesn’t handle message retrieval—this is where POP (Post Office Protocol) and IMAP (Internet Message Access Protocol) come in.

• SMTP is responsible for sending emails from a client to a server or between mail servers. It works in tandem with email retrieval protocols to ensure smooth communication.
• POP allows users to download emails from the mail server to their local device and then typically deletes them from the server. This makes it a good choice for those who prefer offline access.
• IMAP, on the other hand, keeps emails stored on the server, allowing users to access them from multiple devices while maintaining synchronization.
• ESMTP (Extended SMTP) is an enhancement of SMTP, introducing additional features such as authentication and encryption, improving security and deliverability.

Understanding these protocols helps in optimizing email infrastructure for better performance and reliability.

SMTP is necessary for your email program, but that doesn’t mean it needs to be expensive, or overly complicated. Check out our free solution to test out our solution and learn more about your SMTP options.