The Simple Mail Transfer Protocol (SMTP) is a technical standard for sending emails over the internet.
It allows computers and servers to exchange data easily, regardless of their underlying hardware or software.
Put simply, SMTP is the engine responsible for sending your outgoing email messages from your computer, or your website, to a mail server.
In this setup, your email client interacts directly with an SMTP server to send your emails. The server then processes these outgoing messages for delivery to the recipient’s mail server.
For service-based businesses, configuring a reliable SMTP service is crucial to ensure website form submissions actually reach your team.
Without it, internal notifications frequently end up lost in the spam folder.
It is equally important for ensuring that automated customer notifications, such as eCommerce order confirmation emails, reliably land in the recipient’s inbox.
Using a dedicated SMTP server protects your sender reputation and keeps your transactional emails out of junk folders.
There are also significant performance benefits to offloading your email delivery to a specialised SMTP provider.
It reduces the load on your website hosting environment, allowing your high-performance website to run faster and more efficiently.
You will typically interact with SMTP when setting up a professional inbox for your business too.
Whether you are setting up a custom email with Google Workspace or a custom email with Microsoft 365, correctly configuring your SMTP settings will ensure your emails work without interruption.
How Does SMTP Work?
Transmitting emails from your computer or website to its final destination involves a specific, step-by-step process.
While it happens in a matter of seconds, your email actually passes through several digital checkpoints before reaching the recipient’s email inbox.
Mail User & Submission Agents
The journey begins with your email client, which is technically known as a Mail User Agent (MUA).
When you compose a message and hit send, your MUA pushes the outgoing email to a Mail Submission Agent (MSA).
The MSA receives the initial mail submission, checks the data for any immediate errors, and prepares the message for transfer.
Message Transfer & Mail Delivery Agents
Next, the email is handed off to a Message Transfer Agent (MTA), which acts as the core routing system for the internet.
MTAs communicate with each other strictly using the SMTP protocol to securely transfer the message across different networks.
Once the email finally reaches the recipient’s server, a Mail Delivery Agent (MDA) takes over the process.
The MDA is solely responsible for saving the email message directly into the recipient’s inbox so they can eventually open and read it.
DNS Records & The Mail Exchange Process
To find the correct destination, your server’s MTA must accurately locate the recipient’s domain on the internet.
It achieves this by querying the Domain Name System (DNS) to find specific DNS records associated with the recipient’s email address.
Specifically, the system looks for the Mail Exchange (MX) records, which point directly to the recipient’s mail server.
This routing process relies heavily on accurate DNS settings, making it important to understand the implications of domain propagation whenever you update your domain settings.
Once the correct mail exchange details are verified, the message is routed seamlessly to its final destination.
Common Problems for Sites Without an SMTP Server
Relying on your default website hosting environment to send emails is a common mistake that can easily cost you valuable leads.
Without a dedicated SMTP server, automated emails sent directly from your website are highly likely to be flagged as spam by receiving servers.
This means critical form submissions, customer inquiries or eCommerce order receipts may simply disappear into junk folders unnoticed by your team or contacts.
Standard web servers also frequently lack the advanced sender authentication mechanisms required by modern email providers to verify your identity.
To guarantee successful email delivery, you must implement protocols like SPF, DKIM, and accurately setup an email DMARC policy to prove you are a trusted sender.
Furthermore, processing hundreds of outgoing messages directly on your web host consumes valuable server resources.
Offloading this task to a dedicated SMTP provider ensures your website remains fast and responsive for your visitors.
SMTP vs IMAP & POP
While SMTP is essential for sending communications, it is not a mail retrieval protocol.
Instead, SMTP is strictly responsible for pushing your outgoing emails outward to the mail server.
To actually read the messages sitting in your recipient’s email inbox, their email client must use separate protocols designed to pull data down from the server.
The Internet Message Access Protocol (IMAP) allows users to access and manage their emails directly on the server.
This synchronises your messages across multiple devices, which is ideal for busy professionals working seamlessly between their phones and desktop computers.
Alternatively, the Post Office Protocol (POP) typically downloads emails to a single device and may delete them from the server entirely.
To summarise, SMTP handles your outgoing mail while IMAP or POP handles incoming mail.
Choosing an SMTP Email Server for Your Website
Selecting the right email infrastructure is a critical decision that directly impacts your daily business operations.
If your business already uses Google Workspace or Microsoft 365, you can easily use your existing platform as your website’s primary SMTP server.
This is an excellent, cost-effective starting point for service-based businesses that only need to process a relatively small volume of internal email notifications.
However, for eCommerce stores or high-traffic websites sending frequent customer receipts, we strongly recommend upgrading to a dedicated, cloud based SMTP service.
Platforms like SendLayer or SMTP.com are specifically designed to handle high-volume automated routing and keep your transactional emails out of the spam folder.
Using a specialised email provider proactively protects your primary domain reputation by separating your automated website notifications from your teams email communications.
If you require assistance managing these external tools and technical configurations over time, our website care plans ensure your website remains highly secure and perfectly optimised.
Frequently Asked Questions
What port does SMTP use?
SMTP previously used port 25 for communication across the internet. However, some email service providers now block outgoing traffic on port 25 to prevent spam.
Because of this, it can also use ports 465, 587 and 2525 for secure email transmission. Port 465 is commonly used for SMTP over SSL, while port 587 is used for SMTP with STARTTLS to allow encrypted communication.
What are SMTP commands?
SMTP commands are predefined text-based instructions that tell a client or server what to do and how to handle any accompanying data.
An SMTP session consists of commands originated by an SMTP client and corresponding responses from the server, forming a command/reply sequence.
For example, the HELO command is used by the email client to identify itself to the server and starts the SMTP connection process.
Throughout this sequence, SMTP utilises TCP for reliable message transfer and provides error messages if an email cannot be delivered.
What is ESMTP?
The original SMTP protocol supported only unauthenticated 7-bit ASCII text communications, meaning it was limited to sending plain text messages.
Extended Simple Mail Transfer Protocol (ESMTP) is an enhancement of the original protocol that allows for sending email attachments and multimedia content.
ESMTP introduces additional instructions, such as the EHLO command, which is used to initiate a connection and identify the specific capabilities of the server.
This modern upgrade allows for a richer email experience and supports vital features.
Is SMTP secure?
SMTP originally did not include any authentication mechanisms, making it highly vulnerable to email spoofing and spam.
Today, SMTP Authentication (SMTP AUTH) requires clients to log in with credentials before sending emails to prevent unauthorised use of mail servers.
Modern servers also use the STARTTLS extension, which allows servers to upgrade an existing insecure connection to a secure one using TLS.
Does SMTP receive emails?
No, technically the Simple Mail Transfer Protocol (SMTP) is strictly a standard for transmitting electronic mail over a network.
It is fundamentally different from IMAP, which synchronises messages across multiple devices and POP which downloads emails to a single device.
SMTP is only responsible for pushing your outgoing emails to the internet.
