What Is SMTP? A Simple Explanation for Non-Developers

If a tool in your stack has asked for "SMTP settings," or Claude Code mentioned SMTP while adding email to your project, you probably want to know what it is and whether it changes anything for you.

This guide answers what SMTP is in plain English, where you'll run into it, how it differs from an email API, and when you need to care. There's no code involved.

What is SMTP, in plain English?

SMTP stands for Simple Mail Transfer Protocol. It's the agreed set of rules mail servers follow to pass an email from one to the next until it reaches the right inbox. A protocol is a shared language that lets two computers understand each other.

Think of the postal system. A letter is handed between sorting offices until it lands in the right mailbox, and every handover follows the same rules. SMTP does this for email. The sending server says who the email is from, who it's for, then hands over the message, and the receiving server confirms it has taken responsibility for it.

A few things worth knowing:

• SMTP is about sending. It moves email out and along. Reading email in an inbox uses different standards.
• It's old and everywhere. Today's version, published in 2008, builds on a standard from the early 1980s. Every email service uses SMTP to deliver mail.
• It was built to move mail, not to prove who sent it. That's why sending from your own domain means adding a few records to prove the domain is yours. Our guide to verifying your domain for email covers that step.

Where will you see SMTP settings?

You'll usually meet SMTP as a short form asking for a few values. Here's what each one means.

The port numbers mean different things:

If your email service lists both 587 and 465, either works. The standard says they're equally secure when both sides are set up correctly.

Here are the places these forms tend to show up:

1. Sign-in tools. Supabase Auth sends sign-up confirmations, magic links and password reset emails. Its built-in email server is meant for testing: it only sends to members of your project's team, and currently allows 2 emails per hour. For real users, Supabase asks you to add custom SMTP settings in your project's Authentication settings. After that, it starts with a limit of 30 emails per hour, which you can raise.
2. Email programs. Outlook or Apple Mail ask for an outgoing (SMTP) server when you add a mailbox by hand.
3. Website builders, form tools and plugins. Many have an "SMTP" option so their emails go through a proper email service.

SMTP vs an email API: what's the difference?

Both get your email to an email service. The difference is how your project hands it over.

With SMTP, a tool connects to the email service's server using the host, port, username and password, and passes the email along using the mail rules. Any tool with an SMTP settings box can do this without custom code.

With an email API, your project sends a short request over the web with the email's details and a secret key. An API is a way for software to talk to a service, and an API key is the password that proves the request comes from you. This needs a small piece of sending code in your project, which Claude Code writes.

Either way, SMTP is still involved: once your email reaches the email service, the service uses SMTP to deliver it to the recipient's mail server. So the choice is only about the first handover. For a deeper look at the trade-offs, read SMTP vs email API.

When you ask Claude Code to add email to something you're building, it will often use the email service's API, since it's writing code for your project anyway. You don't need to decide this alone:

I'm setting up emails for my project. Should we send them through an email API or SMTP? Explain the choice in plain English, then go with whichever suits this project best.

Claude Code will explain which fits and why, then set it up.

Do you need to care about SMTP?

Mostly, no. SMTP runs in the background of every email, and you rarely touch it. There are two moments when it matters.

A tool asks you for SMTP settings. This is the common one, often with Supabase Auth before launch. Get the values from your email service, paste them into the form, and send a test to yourself. If the password field wants a secret key, treat it like any other key and keep it out of shared documents.

Supabase says I need custom SMTP before real users can get sign-up and password reset emails. Tell me which value goes in which box, and where to find each one in my email service.

You'll get a field-by-field list you can follow in the Supabase dashboard.

You're adding a mailbox to an email program. Your mailbox provider gives you the SMTP server and port, and the program does the rest.

Otherwise, leave SMTP to the email service. When Claude Code builds your transactional emails, such as welcome emails and receipts, it sets up the handover, and SMTP delivery happens on the email service's side.

If you build with Claude Code and don't write code, SimpleSends is the transactional email service built for you: connect SimpleSends to Claude Code once, then brief it in plain English to design, write and build every email your project sends.