We send emails to one another all the time—a whopping 306.4 billion emails are sent and received every day. It’s one of the most common communication methods for businesses and individuals alike, but have you ever stopped and wondered what goes on after you click “send”? How does your message go from you to your recipients? Show
There’s a complex process going on behind the scenes
to get your emails where they need to be. At the center of it all is an email protocol known as SMTP that’s critical for sending emails… and you’re about to learn everything you need to know about it. Jump to
What is SMTP? #SMTP, which stands for Simple Mail Transfer Protocol, is an email protocol used for sending email messages from one email account to another via the internet. Email protocols are sets of rules that let different email clients and accounts easily exchange information, and SMTP is one of the most common ones alongside POP and IMAP. It is also the only dedicated protocol for sending emails. Most email clients—including Outlook, Apple Mail, Gmail, and Yahoo Mail—rely on SMTP to “push” or send messages from a sender to a recipient. What is an SMTP server? #Like all servers, an SMTP server is an application that provides a service to other applications within a network, called clients. Specifically, an SMTP server handles the sending, receiving, and relaying of email. You can think of servers as your real-life post offices. When you send a letter from city A to city B, it first reaches a local post office in city A; here, it gets processed and sent on the post office in city B, which is in charge of delivering it to its final destination. The same happens with SMTP servers—though instead of taking days, the process takes a few minutes at most. You might have also come across the term SMTP port. Those are the communication endpoints that handle the transfer of email data over SMTP as it moves through a network, from one server to another. We cover those in detail here. How does SMTP work? #The best way to explain how SMTP works is to go over the sending process, the individual rules and commands that power it, and the errors you may encounter. Fair warning: This is where things start to get technical. Still, we’ll do our best to condense this protocol down into easy-to-digest chunks. Once an SMTP server is established, email clients can connect to and communicate with it. When the user hits “send” on an email message, the email client opens an SMTP connection to the server so it can send. (The SMTP connection is built on something called a TCP connection, which stands for Transmission Control Protocol.) From there, the SMTP client uses commands to tell the server what to do and transfer data, like the sender’s email address, the recipient’s email address, and the email’s content. The Mail Transfer Agent or Message Transfer Agent (MTA) checks to see if both email addresses are from the same email domain, such as gmail.com:
You don’t need to get into the weeds of SMTP to start sending emails. Postmark’s SMTP email service can handle it all for you and getting started is as simple as copying some credentials into a config file. Learn more → Basic SMTP commands #As we mentioned earlier, SMTP commands are a set of codes that power the transmission of email messages between servers. Here are the basic SMTP commands you should be aware of:
Picture these commands as the language that allows email servers to have a conversation with each other. Their chat will look somewhat like this: There are other SMTP commands that handle authentication and enhance security, such as AUTH and STARTTLS. If you’re interested in learning about them or seeing examples of SMTP at work, read this guide to SMTP commands. Understanding SMTP error codes #The email sending process doesn’t always go as smoothly as in the example chatter of our email servers above. Bounces, blocks, or other issues might prevent an email from being sent. In this case, the receiving server can notify you of issues using SMTP error codes, and knowing what they mean helps you diagnose and fix email delivery roadblocks. For example, here are two groups of SMTP errors that crop up often:
PS: if you’re interested in learning more about SMTP errors, here at Postmark we started a public SMTP Field Manual to document the error codes we see most often. Check it out! You can even contribute your own entries if you’ve spotted an SMTP error we haven’t encountered before. How is SMTP different from other email protocols? #Cast your mind back to the definition of SMTP, and you’ll remember we said that it was one of many email protocols. POP and IMAP are the other two most common email protocols in use. The main difference between these protocols is that SMTP is the only protocol for sending or “pushing” email from one unknown mail server to another. POP and IMAP are protocols for receiving or “pulling” mail for the recipient from their own mail server. So, POP and IMAP limit the transfer of mail to verified mail servers only. They can’t be used for communication outside of your own networks. Different protocols in the sending process: SMTP is used to send email, POP and IMAP to receive mailBelow, we’ll give you a deeper explanation of how POP and IMAP work and how they differ from SMTP. POP #POP stands for Post Office Protocol, and it’s used to receive incoming messages. The most current version is POP3, last updated in 1988. This protocol gets its name because it operates like a real-life post office in the digital realm. POP3 will receive emails and hold them for clients until they pick them up. All emails are downloaded and stored locally, making it a handy solution for someone who only uses one computer to check emails. It’s also commonly used by businesses so employees can check emails while offline. How do POP and SMTP differ?
IMAP #IMAP stands for Internet Message Access Protocol. In short, IMAP stores messages on an email server, but this server can be accessed by the user to check and configure their emails. The difference between this and POP is that IMAP uses a cloud server so emails can be authenticated and categorized by any device. Many email users prefer IMAP to POP because of the convenience and efficiency. If you’ve ever set up your own email client, you probably encountered IMAP on an interface like this one: How do IMAP and SMTP differ?
Running your own SMTP server vs. using a third-party email service: What’s best? #When it comes to setting up and using an SMTP server, there are two main options:
What’s the best option? There’s no black-and-white answer, as it all depends on your needs and resources. Below, we’ll take a look at both and highlight the pros and cons of each so you can determine which path to go down. Running your own SMTP server: Pros and cons #What sort of advantages or disadvantages can you see from setting up and running your own SMTP server? Here’s an overview, plus a bonus step-by-step guide to SMTP setup and authentication for a more detailed rundown.
Speaking of which... Using a third-party email service: Pros and cons #Now, admittedly we are going to be a bit partial here, because at Postmark we do run a super-reliable email platform that you can use to deliver email via SMTP. This said, we also know that Postmark and other
third-party services are not for everyone—so here is a list of the advantages and drawbacks of opting for a third-party email service.
Pro tip: you can check the reputation of your provider on review sites like G2 or by checking out conversations on Twitter and other social channels. Real feedback from actual product users might give you a good idea of what to expect. Aww. Thanks Val. 💛
Overall, opting for a third-party email service is the easiest and most
reliable way to go for most businesses. It equals fewer risks and fewer headaches, allowing you to focus on your email strategy and send emails without worrying about the technical side of things. Choose an email service you can count on #Here’s the bottom line: The world’s email runs on SMTP, and while you can definitely set up your own SMTP server, it may end up being a bigger challenge than it needs to be—particularly if you have no prior knowledge of the inner workings of email protocols, don’t have a lot of resources, and are concerned about security. And even though here at Postmark we’re all excited about designing the perfect email infrastructure, we understand that most normal humans would rather not deal with it. Most of the time, finding a third-party email service is the best bet, and if you choose Postmark you’ll have an email service you can truly count on. Our SMTP servers are distributed around the globe, giving you access to easy and secure email delivery at all times. And because we have serious street cred with inbox providers, we’re getting your emails delivered fast and reliably. The best SMTP providers: A side-by-side comparison #Looking for the best SMTP email service for your business? We compare features, costs, and ideal use cases of popular tools and share a handy comparison sheet that might help you as you do your research. Are you looking for a side-by-side comparison of the best SMTP providers? Here's a breakdown.Ready to start sending?Sign up for Postmark today and enjoy a free trial on us. Start a free trial → No credit card required Learn more about SMTP sending with Postmark:
What is SMTP and IMAP?SMTP is the protocol used to send emails between clients and servers or between servers. On the other hand, IMAP is a protocol for managing and retrieving emails from a server.
Which protocol provides access to an inbox stored in an email server?Internet Message Access Protocol, or IMAP, is a standard email retrieval (incoming) protocol. It stores email messages on a mail server and enables the recipient to view and manipulate them as though they were stored locally on their device(s).
Is POP3 or IMAP better?IMAP is better if you are going to be accessing your email from multiple devices, such as a work computer and a smart phone. POP3 works better if you are only using one device, but have a very large number of emails. It is also better if you have a poor internet connection and need to access your emails offline.
What is POP3 used for?POP3 (Post Office Protocol)
POP3 is an older protocol that was originally designed to be used on only one computer. Unlike modern protocols that use two-way synchronization, POP3 only supports one-way email synchronization, only allowing users to download emails from a server to a client.
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