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What is SMTP and how does it work?

Ever wonder what's going on behind the scenes of your email sending? If you're curious about the protocol driving your email exchange, you're in the right place. SMTP is used to send and receive emails and handle message transmissions between servers. Here's everything you need to know.



SMTP, though a pillar of email delivery, often gets lost in the jumble of tech terms and acronyms. But if you're ready to send impactful emails, this is one of those acronyms that you should take time to understand. Let's dive into what SMTP is and why it's your key to a successful email program.

What is SMTP?

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.

How Does SMTP Work?

Hitting send marks the end of your email process as a human sending a message, but that’s when SMTP springs into action. Here's a high-level breakdown of how SMTP works:

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.

Learn more about SMTP ports in our post on which SMTP port should you use?

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.

How SMTP works

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

A quick word on IMAP

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.

A deeper dive into SMTP

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)

Are you into the tech side of email? Welcome fellow email geek! If you find the breakdown of SMTP interesting, you may enjoy this post on how email works.

Why is SMTP important?

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.

Configuring SMTP

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.

Can you configure your own SMTP server?

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.

What are the security threats of using a built-in SMTP server?

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.

What are SMTP commands?

Outside of understanding SMTP basics – and how SMTP can improve deliverability – you need 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 a big part in the efficiency and reliability of email delivery across the internet. SMTP commands 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 email content.

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

Learn more about SMTP commands and which commands to use in our post on understanding SMTP commands.

Why are SMTP service providers important?

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.

Learn more about email authentications in our post on email authentication basics.

How do SMTP service providers help maintain and set up email infrastructure?

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.

How do SMTP service providers improve my email delivery speeds?

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.

How do SMTP service providers improve deliverability?

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.

What are some SMTP best practices?

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.

Wrapping up

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.

Learn more about SMTP with Sinch Mailgun

Free SMTP relay service

Particularly for growing businesses, you want access to powerful features, real-time analytics, and the ability to scale as both your email marketing and outgoing mail multiplies. That flexibility is at the core of Mailgun so as your business and marketing campaigns evolve, your free SMTP service doesn’t have to change.

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