The average worker receives 117 emails per day, according to Microsoft. With so much email delivered to inboxes, it’s easy to overlook the system that powers this communication. One key piece of that system is Simple Mail Transfer Protocol (SMTP), the protocol that ensures messages are sent and delivered reliably. 

For marketers and businesses that use email to engage customers, promote products and services, and drive sales, understanding how SMTP works can be invaluable. Knowing how this process functions can help ensure your critical messages land in inboxes and not spam folders. 

This guide breaks down the message-sending and delivery protocol at the heart of email, exploring what it is, how it differs from other email protocols, and how it works.

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is a set of communication guidelines—a software rulebook, so to speak—for transferring email messages between mail servers. It’s essential for reliable delivery, as it verifies the sender, routes the message to the right place, and hands it off to the mail server that handles the recipient’s email. SMTP is a push protocol, meaning it deals with sending messages and not retrieving them. Without it, however, your email campaigns designed to engage customers and drive sales wind up as dead letters or sucked into the spammy black hole, never to be seen again. 

Over time, SMTP has evolved to address security and email deliverability concerns. For example, extensions help verify senders (SMTP AUTH), encrypt messages in transit (STARTTLS), and report delivery outcomes known as Delivery Status Notifications (DSN).

SMTP vs. IMAP vs. POP3: What’s the difference?

Email delivery involves multiple communication protocols—SMTP, IMAP, and POP3—which can be confusing at first. However, each serves a distinct purpose: SMTP handles the sending and relaying of emails, while IMAP and POP3 are responsible for receiving those messages and delivering them to your inbox. Here’s how they differ: 

• SMTP (Simple Mail Transfer Protocol) is the standard protocol used for sending emails. When you compose and send a message from your email client, SMTP transfers that message to the recipient’s email server. It doesn’t handle incoming mail—that’s the job of IMAP or POP3—so SMTP strictly deals with outgoing email. 
• Internet Message Access Protocol (IMAP) lets your email client, like Gmail or Outlook, access messages stored on the mail server without downloading them permanently. Because you are viewing a synced copy, you can access the same email from multiple devices. Changes made on one device, like marking an email as read or deleting it, sync across all devices. This makes IMAP good for people on the move who check email from many devices (desktop, phone, tablet, browser).
• Post Office Protocol version 3 (POP3) is an older protocol that uses less server storage. It downloads emails from the mail server to a single device, generally deleting them from the server. As a result, messages are stored locally and may not be accessible from other devices, so this protocol is better for users who check email from just one location or computer.

What is an SMTP server?

Acting as a postal service for email, an SMTP server—also known as an outgoing mail server—is the software that uses Simple Mail Transfer Protocol guidelines to ensure outgoing messages get where they need to go. When you send an email from a mail client, the client communicates with the SMTP server, which then connects with other mail servers on the internet to deliver the message. From the moment you hit Send on a promotional newsletter or on automated transactional emails confirming a purchase, SMTP servers manage logistics, handle authentication, negotiate the handoff between servers, and work with message transfer agents (MTAs) to route your messages properly to intended recipients.

SMTP servers can be free (generic/shared) or dedicated. Free SMTP servers—like those included with personal email accounts—work just fine for casual personal emailing. They may have daily sending limits and shared IP addresses (the machine-readable numbers that browsers use to retrieve a site’s content), which can hurt deliverability and sender reputation. As such, businesses that send a lot of emails—like order confirmations, newsletters, or promotional campaigns—can benefit from paying for a dedicated SMTP server. Dedicated servers provide better performance, improved email deliverability, customizable authentication, and access to analytics and support.

Popular SMTP server providers

• SendGrid
• Mailgun
• Amazon Simple Email Service (SES)
• Postmark
• SMTP.com
• Shopify email services

If you’re looking for a dedicated SMTP solution, some of the top providers include the following:

SendGrid

SendGrid’s cloud-based SMTP service manages high-volume transactional and marketing emails. It offers SMTP relay, email API, analytics, Sender Policy Framework (SPF) authentication, ISP reputation monitoring, and feedback loops, in which ISPs notify senders when recipients mark emails as spam. 

Pricing: Starts at $19.95 per month for 50,000 to 100,000 emails.

Mailgun

Mailgun has powerful application programming interfaces (APIs) for embedding email sending and receiving functionality within your applications. It’s popular with developers for its flexible handling of transactional emails, marketing campaigns, and incoming messages. 

Pricing: Mailgun offers a free plan allowing 100 emails per day; paid plans start at $15 per month for 10,000 emails.

Amazon Simple Email Service (SES)

Amazon SES is a cloud-based email sending service provided by Amazon Web Services (AWS). It’s good at handling high-volume bulk emails and is known for high scalability and reliability. Amazon SES supports dedicated, shared, or owned IP address deployments and includes analytics and reporting. 

Pricing: It offers pay-as-you-go pricing, with outbound messages costing 10¢ per 1,000 emails plus 12¢ per gigabyte of data in the attachments sent.

Postmark

Postmark is an email delivery service focusing on transactional emails, such as password resets and order confirmations. It emphasizes speedy, reliable delivery of time-sensitive messages and includes analytics and a library of prebuilt but customizable email templates. 

Pricing: Postmark’s plans start at $15 per month for 10,000 emails.

SMTP.com

SMTP.com is a delivery service provider for any size business, with SMTP relay (forwarding email messages from one mail server to another) and API-based delivery. It offers advanced insights, email list monitoring, and domain and IP reputation safeguarding as add-ons to your subscription. 

Pricing: SMTP.com’s entry-level plan is $25 per month for 50,000 emails.

Shopify email services

Shopify doesn’t offer a built-in SMTP service for sending emails directly from your server. Instead, it manages transactional emails, such as order confirmations and shipping updates, through its own email hosting system. You don’t need to configure an SMTP email server within your core Shopify settings. You can tailor particular communication preferences with these other email options: 

• Shopify Email app. The Shopify Email app is useful for creating email marketing campaigns and custom workflow automations directly from Shopify. Every month, you can send up to 10,000 manual or automated emails for free, and pay as you go after that.
• Email forwarding. If you have a Shopify-managed custom domain, you can set up email forwarding so messages sent to a custom address like [email protected] are forwarded to your personal inbox (e.g., Gmail or Outlook). 
• Third-party email hosting. If you want to use a third-party email marketing service for more control over deliverability and branding, you can set up connected email hosting. This allows the third-party service to send emails on your behalf, using their SMTP server and enhanced features.
• Third-party apps. Some third-party Shopify apps support custom SMTP. For example, certain multivendor apps let you connect your preferred SMTP provider, giving you full control over sender identity and deliverability. 

How does an SMTP server work?

Email communication can seem complicated. Here is a simplified process of an email message’s transmission:

1. The email client initiates a connection to an SMTP server. When you hit the Send button in an email client like Outlook or Apple Mail, it connects to an SMTP server, generally over SMTP port 587 for secure submission. The client sends the details—recipient, subject, and body—to the SMTP server. If you’re using an email service provider (ESP) to send in bulk, the ESP software acts as the client, connecting to its own SMTP infrastructure.

2. The SMTP server checks addresses and performs a Domain Name System (DNS) lookup. The SMTP server processes the email details from the client, checking the sender and recipient addresses. Then it performs a DNS record lookup to find the mail exchange (MX) record for the recipient’s domain. The MX record tells the SMTP server where to send the email—like finding the right postal address for an envelope. 

3. The SMTP server relays the message. The SMTP server pushes the message from server to server like a relay race or chain of digital post offices. Along the way, servers check the sender’s authenticity and recipient’s validity. For email marketing, this is where reputation plays a make-or-break role. If your ESP has a poor reputation, the recipient server might reject your email, leading to bounces or messages being sucked into spam folders. In some high-volume or high-availability environments, a backup server may also be configured to step in if the primary SMTP server fails.

4. The recipient’s server receives and stores the email. Once the email is successfully transferred to the recipient’s server, the SMTP server’s job is done. The receiving server performs its own checks (spam filters, antivirus scans) and stores the message. At that point, the recipient accesses the email via their email client, using IMAP or POP3 to download or sync the message to their inbox.