What Is a Transactional Email Service? Plain-English Guide

So what is a transactional email service? It's a company that delivers the automatic emails your project sends to one person at a time, like a password reset or a receipt. Your SaaS or site tells the service who to email and what to say, and the service gets it to their inbox.

If Claude Code has told you to "set up a transactional email service", this guide explains what that means, why your project needs one, and how to pick one. It's written in plain English, without developer jargon.

What is a transactional email service? The short answer

A transactional email service is the delivery company behind your project's automatic emails. When something happens in your SaaS, like a new sign-up, your project hands the email to the service, and the service sends it from your own address, such as hello@yourcompany.com.

A transactional email is any email triggered by something one person did, or by a change to their account. Here are some common ones:

For the full set, with what to put in each one, see 12 transactional emails every SaaS needs. If you're not sure your project needs any of this yet, read Claude Code told me I need a transactional email service. Do I?

What's the difference between transactional and marketing email?

Transactional emails are triggered by one person and sent to that person. Marketing emails, like newsletters and promotions, go out to a list of people to promote something.

The US Federal Trade Commission (FTC) draws the same line in its CAN-SPAM guidance. It treats an email as transactional when its main job is to complete or confirm a purchase the person already agreed to, give them safety or security information, or tell them about a change to their account or subscription. Purely transactional emails are exempt from most CAN-SPAM rules, though they still can't use false or misleading sender details.

The FTC also says an email that leads with promotions can count as marketing, so keep each transactional email focused on its task.

Our guide to transactional vs marketing emails goes deeper. SimpleSends is a transactional email service, so the rest of this guide is about the first column.

Why can't your project send email on its own?

In practice, email your project sends by itself gets blocked or treated as suspicious, for two reasons.

Hosting companies block it. Email travels between servers using a standard called SMTP (Simple Mail Transfer Protocol). Servers traditionally hand mail to each other through a channel called port 25, and spammers abused it for years. So many hosts block it. Google Cloud, for example, blocks outgoing connections on port 25 to anywhere outside your own network, and points customers to third-party email services instead.

Inboxes want proof. Since February 2024, Gmail has required everyone who sends to personal Gmail accounts to set up SPF or DKIM. These are short records added to your domain's DNS settings (the address book that tells the internet where your domain's services live). They prove an email really came from you. Gmail also requires an encrypted connection and a few other technical checks. Emails that fail can be rejected or sent to spam.

A transactional email service already runs servers that meet these requirements. You connect your domain to it once, and from then on your project sends through it.

What does a transactional email service do for you?

The service handles everything between "your project wants to send an email" and "the email arrives". That includes:

• Sending each email from your own domain, the moment your project asks.
• Proving the email is from you. This step is called domain verification: you add a few DNS records where your domain is registered, and the service checks them. Our guide to verifying a domain for email walks through it.
• Handling bounces, so your project knows when an address doesn't exist.
• Keeping logs of every email: when it was sent, and whether it was delivered or bounced.
• Keeping access private with a secret API key. An API key works like a password your project uses to prove it's allowed to send.

Every transactional email service needs the same building blocks: an account, a verified domain, a secret key stored in your project's hosting settings, and a small piece of sending code in your project. Claude Code writes the sending code, so you don't write any code yourself.

How do you choose a transactional email service?

Choose based on who the service is built for and what working with it is like day to day.

If you write code yourself, or want to edit the code yourself, a developer-focused service like Resend or Postmark is a strong fit. The same goes if you send very high volumes. For a side-by-side look at the options, see the best transactional email services for beginners.

Where does SimpleSends fit?

SimpleSends is a transactional email service built for people who build with Claude Code but don't write code. Setup uses the same building blocks as any other service. The difference is what working with it is like after you sign up:

• Everything happens in the chat. You don't need a dashboard for day-to-day work. You brief Claude Code the way you'd brief a person.
• Claude Code does the whole job. It designs, writes, builds and changes every transactional email, from the welcome email to receipts.
• Problems get sorted in plain English. Claude Code can read your send logs and delivery status through SimpleSends, tell you whether an email went out, and help work out why one didn't.

SimpleSends links to Claude Code through a connector. A connector uses MCP (Model Context Protocol), an open standard that lets AI tools work with other services; what is MCP? explains it in more detail. Claude Code uses the connector while you build, to set things up, send test emails and check logs. Once your project is live, real emails go from your project to SimpleSends directly, whether or not Claude Code is open.

How do you connect SimpleSends to Claude Code?

You sign up once, connect once, and then work in the chat. You'll need these two settings in step 2:

1. Sign up on the SimpleSends website and follow the steps there to set up the email address you'll send from. To send from your own domain, this includes adding a few DNS records where your domain is registered.
2. In Claude Code, open Customize, then Connectors, choose Add, then Add custom connector. Enter the name and server URL from the table above.
3. When Claude prompts you to connect, log in to SimpleSends.
4. Back in the Claude Code chat, send the message below.

I've connected SimpleSends for all transactional email.

Claude Code takes it from there, including adding what your project needs to send email. Then tell it what you'd like:

Please set up all the transactional emails my SaaS needs. Start by suggesting which ones I should have and why, and wait for me to approve the list before you build anything.

You'll get a proposed list to review before Claude Code designs and writes each email.