How do I send transactional emails from my app with an API?

To send transactional emails from your app with an API, you typically use an email service provider (ESP) like SendGrid, Mailgun, Postmark, Amazon SES, or Resend.

Basic flow

1. Create an account with an email API provider.
2. Verify your sending domain so emails aren’t flagged as spam.
3. Get an API key from the provider.
4. Call the provider’s API from your backend when an event happens, like:
   • user signs up
   • password reset requested
   • receipt/invoice generated
   • email verification needed
5. Use templates for consistent transactional messages.
6. Handle errors and retries in your app.

Example API request

Most providers use a POST request like this:

curl -X POST "https://api.provider.com/send" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "Your App <noreply@yourdomain.com>",
    "to": "user@example.com",
    "subject": "Reset your password",
    "html": "<p>Click here to reset your password.</p>"
  }'

Example in Node.js

import fetch from "node-fetch";

async function sendEmail() {
  const res = await fetch("https://api.provider.com/send", {
    method: "POST",
    headers: {
      "Authorization": `Bearer ${process.env.EMAIL_API_KEY}`,
      "Content-Type": "application/json"
    },
    body: JSON.stringify({
      from: "Your App <noreply@yourdomain.com>",
      to: "user@example.com",
      subject: "Welcome!",
      html: "<p>Thanks for signing up.</p>"
    })
  });

  if (!res.ok) {
    throw new Error(`Email send failed: ${res.status}`);
  }

  return await res.json();
}

Good practices

• Send from backend only: never expose API keys in frontend code.
• Use templates for password resets, receipts, etc.
• Keep transactional emails separate from marketing emails.
• Set SPF, DKIM, and DMARC records for your domain.
• Log delivery status and failures.
• Use webhooks if the provider supports bounce/open/delivery events.

Choosing a provider

• Postmark: excellent for transactional email
• SendGrid: flexible, widely used
• Mailgun: developer-friendly
• Amazon SES: low cost, more setup
• Resend: modern developer experience

If you want, I can show you:

• a working example in your backend language
• how to set up domain authentication
• how to send templated emails with one specific provider