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.
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.
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.
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, and Twilio SendGrid’s is smtp.sendgrid.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.
SMTP email services handle all the SMTP protocol details for you. They're email service providers (like Twilio SendGrid) 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:
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 butcan be an application constantly running in anticipation of sending new mail.
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.
Your mail client connects to the SendGrid SMTP server, smtp.sendgrid.net, using port 25 (by default).
Your mail client has a conversation with our server, first verifying that the username and password used for authentication correspond to an active account, then relaying the message information, such as sender address, recipient, and message content.
Our server takes the message information we gathered from your request, then repeats the conversation process in step 2 with the recipient’s mail server.
Your recipient’s mail server checks the sending address, recipient address (to ensure it’s a valid recipient), and message content. It also checks the sending domain for any potential DNS issues, such as invalid DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF) signatures. As long as there are no issues, the recipient’s mail server will use the protocols POP3 or IMAP to retrieve the email and deliver the message to its intended recipient.
Take a look at the diagram below for a visual of the SMTP email delivery process.
SendGrid offers free accounts that you can enable without a credit card. With a free SendGrid account, we give you access to the SendGrid SMTP server and allow you to send up to 100 emails per day—effectively giving you an SMTP server for testing.
Active free account customers can also use their account to send test messages and test their email deliverability before upgrading and sending significantly more email.
All you need is an active SendGrid account and the ability to use basic username and password authentication within your email client or application to send an email using our SMTP server.
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.
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.
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.
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:
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.
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.
While SMTP handles sending emails, POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) are responsible for receiving emails. SMTP pushes emails out, while POP3 and IMAP pull emails into your inbox. POP3 typically downloads emails to a single device, removing them from the server, while IMAP syncs emails across multiple devices by leaving them on the server.
Technically, yes—you can set up your own SMTP server. However, this isn't recommended for most users or businesses. Running your own SMTP server requires technical expertise, constant maintenance, and can lead to deliverability issues. It's generally more efficient and reliable to use an established email service provider that handles SMTP for you.
SMTP is the basic protocol for sending emails, while SMTP relay is a service that routes emails through a trusted third-party server. SMTP relay is particularly useful for businesses sending large volumes of emails, as it can improve deliverability and provide additional features like analytics and security measures.
SMTP authentication (often called SMTP AUTH) is a security measure that requires users to log in with a username and password before sending emails. This helps prevent unauthorized use of mail servers and reduces spam. Common authentication methods include LOGIN, PLAIN, and CRAM-MD5.
SMTP's original design didn't include strong sender authentication, which made it vulnerable to email spoofing—where attackers fake the 'From' address. This weakness is often exploited in phishing attacks. Modern authentication protocols like SPF, DKIM, and DMARC work alongside SMTP to verify sender identities and reduce these threats.
Originally, SMTP was designed for plain text messages only. However, extensions like MIME (Multipurpose Internet Mail Extensions) allow SMTP to handle attachments, HTML formatting, and other rich media. These extensions essentially encode non-text content so it can be transmitted via SMTP.
Now that you're an SMTP expert (basically, right?), it's time to put your knowledge into action. Whether you're a seasoned email marketer or just starting out, Twilio SendGrid has the tools and support you need to make and deliver your email campaigns.
Why choose SendGrid for your SMTP sending?
Reliability: Our robust infrastructure handles billions of emails monthly, guaranteeing your messages get delivered.
Scalability: From 100 emails to 100 million, we grow with your business.
Expert support: Our team of email experts is always ready to help you optimize your sending.
Advanced analytics: Gain deep insights into your email performance to continually improve your strategies.
