Ever wonder what happens behind the scenes when you hit “Send” on an email? Enter SMTP—the Simple Mail Transfer Protocol. It’s the silent workhorse that makes sure your messages travel across the wide expanse of the internet and land in the right inbox.
Think of SMTP as the world’s most efficient postal service. It doesn’t just deliver your email—it plans the route, handles any detours, and makes sure your message arrives safe and sound. And the best part? It does all this in the blink of an eye.
However, exploring the world of email protocols can feel like trying to read a map in the dark. That’s why we’re here to shed some light on SMTP. We’ve rounded up the most common questions about SMTP servers and broken them down into bite-sized, easy-to-digest answers.
What is SMTP (and what does SMTP stand for)?
SMTP stands for Simple Mail Transfer Protocol—it’s an application used by mail servers to send, receive, and relay outgoing email between senders and receivers.
As the technology behind email communication, SMTP is the protocol that allows you to send and receive emails. Without it, email communication would be nonexistent since SMTP determines which servers will receive your relay messages.
- S = Simple: SMTP is designed to be straightforward and easy to implement.
- M = Mail: SMTP is built to send email messages across networks.
- T = Transfer: SMTP is responsible for transferring email messages from one server to another.
- P = Protocol: SMTP is a set of rules (or protocols) that lets different email systems and servers communicate with each other.
SMTP is the digital postal service of the internet. It determines which servers will receive your messages and how they’ll get there. Without SMTP, your emails would be like letters without addresses—they’d have nowhere to go.
What SMTP does (and doesn’t) do
As the “T” in its name indicates, SMTP is a transport protocol: All it does is move messages from point A to point B.
Like many other Internet protocols, SMTP is intended to be used on top of the Transmission Control Protocol (TCP), which guarantees reliable delivery of the individual packets that make up a conversation.
The use of IP means that we can depend on SMTP to eventually get the message contents to a server, but what happens to it after that is up to the server.
Imagine what happens when a delivery driver leaves a package at your office on a Saturday. What happens to it after delivery isn’t his problem.
Deliverability, monitoring, tracking, authentication, and encryption are all examples of services that SMTP doesn’t necessarily provide itself but that are still very valuable.
What is an email server—and what is an SMTP server?
An outgoing mail server is a generalized term to describe a system that collects, processes, and serves email. Much like a mail carrier, every email message passes through the mail server before reaching its destination. Without servers, you would only be able to send emails to people whose addresses matched your domain—Gmail.com to Gmail.com, for instance.
Because an SMTP email server pertains to outgoing email, it’ll have an address (or addresses) that the mail client or application you use can set that generally looks like this: smtp.serveraddress.com. For example, the SMTP server Gmail uses is smtp.gmail.com. You can generally find your SMTP email server address in the account or settings section of your mail client.
Using a store and forward process, SMTP works with the mail transfer agent to move your email across networks to the right computer and email inbox.
So when you send an email with SMTP host Gmail or AOL, the SMTP server processes your email, decides which server to send the message to and relays the message to that email server. The recipient’s inbox service provider, such as Gmail or AOL, then downloads the message and places it in the recipient’s inbox.
You can find more details to help you understand the SMTP meaning on our docs page.
What is an SMTP email service?
SMTP email services handle all the SMTP protocol details for you. They’re email service providers that do all the behind-the-scenes work to get your emails corretlyed formatted, sent, and delivered to the recipient’s mail server.
An SMTP email service manages the complex infrastructure for you. It does everything from:
- Improving email deliverability
- Securing sent emails
- Providing valuable email performance analytics.
Is an SMTP server the same as a normal server?
Like most servers, the SMTP server processes data sent to another server, but it has the specific purpose of processing data related to the sending and relaying of email. An SMTP server also isn’t necessarily on a machine but can be an application constantly running in anticipation of sending new mail.
Why are SMTP servers important?
When communicating over email, it’s crucial to send email to your recipient safely and on time. However, without an SMTP server, your email can’t make it to its destination.
Once you hit Send, the email transforms into a string of code sent to the SMTP server, which processes that code to pass the message. Without the SMTP server to process the message, the message wouldn’t translate.
Additionally, the SMTP server verifies that the outgoing email is from an active account. In other words, the email server checks whether an active or working email account sent the outgoing message, protecting your inbox from illegitimate email.
If the email is undeliverable, the SMTP ensures the email will bounce back to the sender, letting them know they have an incorrect email address or the receiving email server blocked the email for whatever reason.
What SMTP server port do I need to connect to?
We support unencrypted and TLS encryption on ports 25, 587, and 2525. If you’d like to implement SSL encryption, use port 465.
Short history of SMTP
SMTP was built to be a lightweight, easy-to-implement means to transfer plain-text messages between devices. The original SMTP standard distinguished between a mail transport agent (MTA)—what modern administrators would call a server—and mail user agents (MUA), which most people refer to as clients.
SMTP’s goal was to provide a way for two MTAs to pass message traffic back and forth. The protocol doesn’t define any standards for storing messages, giving clients access to them, or carrying complex content types such as audio or image data. It’s just a set of rules for moving a properly structured message from point A to point B.
ARPANET
The original Internet grew from ARPANET, a closed research network that was designed for academic and government use.
ARPANET and the early implementation of the Internet were designed around the assumption that only a limited number of trusted users would be able to connect to the network.
The competing X.400 protocol was intended to be a more robust message transfer protocol that would be suitable for commercial use; X.400 included features for guaranteed reliable delivery, message tracking, and other security and authentication features. However, it was complex to implement, and it ultimately lost out to the simpler, less complex, less expensive SMTP implementations.
The version of SMTP that we use today still shows a number of design influences that trace back to its origins:
- The protocol itself didn’t provide for authentication or encryption, which weren’t especially important on a trusted network with trustworthy users. (Since SMTP’s appearance, a series of extensions to the standard have added these features.)
- The ability to get return receipts or delivery notifications is dependent on which client recipients use; servers are required to send delivery status notifications (DSNs) only for messages that are permanently undeliverable.
- There’s no enforced mechanism for classes of service; for example, FedEx and UPS both let you choose from multiple delivery speeds according to how much you’re willing to pay.
- Email filtering often seems capricious, and users have learned through experience that important messages may be filtered arbitrarily by the spam filters in their client, in their server, or somewhere in the cloud. (And now of course, recipients can filter messages within their own inboxes through features like Gmail’s “tabs”.)
- SMTP (or email in general) often falls back to using web-based technologies (i.e. img and anchor tags) to archive click and open tracking metrics.
The Evolution of email
As the Internet spread, businesses were allowed to connect to it. However, they were informally banned from using it for commercial purposes.
A few large companies (notably IBM, DEC, and Control Data) built corporate email systems for large-company use, and as time passed, smaller companies—including Novell, Lotus (now IBM Lotus), and Microsoft—saw the business value in email systems and began building and delivering intra-company systems that broadened the email market.
None of these systems provided much in the way of interoperability, although after the SMTP standard was defined a few forward-looking vendors began adding SMTP support through gateways or connectors that linked MTAs from different vendors.
The difficulty of getting interoperability to reliably work across multiple MTA vendors and their individual connectors put a lot of market pressure on vendors to come up with a simpler, more robust solution, so native SMTP began to gather real momentum in the mid-1990s.
Now, it is the de facto standard for transferring mail between servers. Even products that once used proprietary protocols rely on SMTP both for message transport.
SMTP’s evolving role in modern email infrastructure
While SMTP remains the backbone of email delivery, it now works in concert with several newer technologies to improve email security, deliverability, and user experience. Let’s explore how SMTP interacts with some of these modern email innovations:
SMTP and DMARC (Domain-based Message Authentication, Reporting, and Conformance)
DMARC builds upon SMTP by adding an extra layer of sender authentication:
- SMTP handles the actual email transmission, while DMARC verifies that the sender is authorized to use the sending domain.
- When an email is sent via SMTP, the receiving server checks the DMARC record in the sender’s DNS.
- DMARC uses SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail)—both of which work alongside SMTP—to authenticate the sender.
- If authentication fails, DMARC policies instruct the receiving server on how to handle the message, potentially rejecting it before it enters the recipient’s inbox.
SMTP and BIMI (Brand Indicators for Message Identification)
BIMI improves the visual aspect of emails delivered via SMTP:
- After an email is successfully delivered through SMTP and passes DMARC authentication, BIMI comes into play.
- BIMI allows organizations to display their logo next to authenticated emails in the recipient’s inbox.
- This visual indicator helps recipients quickly identify legitimate emails, adding a layer of trust to SMTP-delivered messages.
SMTP and AI-powered email optimization
Modern AI tools are improving how SMTP is used for email delivery:
- AI can analyze SMTP server responses and delivery patterns to optimize sending times and improve deliverability.
- Machine learning algorithms can predict and prevent potential SMTP-related issues (such as bounces or delays) before they occur.
- Smart routing systems can dynamically choose the best SMTP server path for each email to increase the chances of successful delivery.
By working in tandem with these newer technologies, SMTP continues to evolve, helping email remain a secure, reliable, and increasingly intelligent communication channel.
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