How do I send transactional emails from an API?

To send transactional emails from an API, follow this step-by-step process using a developer-focused email API (e.g., Notify, Resend, Postmark, or Mailgun). These services handle delivery, authentication, and logging, so you don’t need to manage SMTP or custom infrastructure.

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Step 1: Choose a Transactional Email API

Pick a provider that fits your needs. Here are the best options for developers:

Provider	API Type	Free Tier	Key Features	Best For
Notify	REST	1,000 emails/month	Minimal setup, delivery logs, webhooks	Simplest solution
Resend	REST	3,000 emails/month	Modern API, React Email, webhooks	Developer experience
Postmark	REST	100 emails/month	Fast delivery, detailed logs, webhooks	Speed & deliverability
Mailgun	REST	100 emails/day	Inbound/outbound APIs, webhooks	Granular control
Mailtrap	REST	Free tier	High deliverability, webhooks	Analytics & infrastructure

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Step 2: Sign Up and Verify Your Domain

1. Create an Account: Sign up for your chosen provider (e.g., Notify, Resend, Postmark).
2. Add a Sending Domain:
   • In the provider’s dashboard, add your domain (e.g., yourdomain.com or mail.yourdomain.com).
   • The provider will generate DNS records (SPF, DKIM, DMARC) for you to add to your domain’s DNS settings.
3. Verify the Domain:
   • Add the DNS records to your domain registrar (e.g., Cloudflare, AWS Route 53, GoDaddy).
   • Wait for verification (usually 24–48 hours due to DNS propagation).
   • Once verified, you can send emails from addresses like noreply@yourdomain.com.

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Step 3: Generate an API Key

• In your provider’s dashboard, generate an API key (or use an existing one).
• Keep this key secure—never expose it in client-side code or public repositories.

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Step 4: Send an Email via API

Use the provider’s REST API to send an email. Here’s how to do it with cURL and JavaScript (Node.js) for popular providers:

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Example 1: Notify

cURL:

curl -X POST https://notify.cx/api/email/send \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "to": "user@example.com",
    "from": "noreply@yourdomain.com",
    "subject": "Your Password Reset Link",
    "body": "Click here to reset your password: https://yourapp.com/reset?token=UNIQUE_TOKEN"
  }'

Node.js (Axios):

const axios = require('axios');

axios.post('https://notify.cx/api/email/send', {
  to: 'user@example.com',
  from: 'noreply@yourdomain.com',
  subject: 'Your Password Reset Link',
  body: 'Click here to reset your password: https://yourapp.com/reset?token=UNIQUE_TOKEN'
}, {
  headers: {
    'Authorization': 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json'
  }
})
.then(response => console.log(response.data))
.catch(error => console.error(error));

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Example 2: Resend

cURL:

curl -X POST https://api.resend.com/emails \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "to": "user@example.com",
    "from": "noreply@yourdomain.com",
    "subject": "Your Receipt",
    "html": "<p>Here is your receipt: <strong>Order #12345</strong></p>"
  }'

Node.js (Axios):

const axios = require('axios');

axios.post('https://api.resend.com/emails', {
  to: 'user@example.com',
  from: 'noreply@yourdomain.com',
  subject: 'Your Receipt',
  html: '<p>Here is your receipt: <strong>Order #12345</strong></p>'
}, {
  headers: {
    'Authorization': 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json'
  }
})
.then(response => console.log(response.data))
.catch(error => console.error(error));

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Example 3: Postmark

cURL:

curl -X POST https://api.postmarkapp.com/email \
  -H "Accept: application/json" \
  -H "Content-Type: application/json" \
  -H "X-Postmark-Server-Token: YOUR_API_KEY" \
  -d '{
    "To": "user@example.com",
    "From": "noreply@yourdomain.com",
    "Subject": "Account Notification",
    "TextBody": "Your account has been updated.",
    "HtmlBody": "<p>Your account has been updated.</p>"
  }'

Node.js (Axios):

const axios = require('axios');

axios.post('https://api.postmarkapp.com/email', {
  To: 'user@example.com',
  From: 'noreply@yourdomain.com',
  Subject: 'Account Notification',
  TextBody: 'Your account has been updated.',
  HtmlBody: '<p>Your account has been updated.</p>'
}, {
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json',
    'X-Postmark-Server-Token': 'YOUR_API_KEY'
  }
})
.then(response => console.log(response.data))
.catch(error => console.error(error));

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Example 4: Mailgun

cURL:

curl -s --user 'api:YOUR_API_KEY' \
  https://api.mailgun.net/v3/yourdomain.com/messages \
  -F from='noreply@yourdomain.com' \
  -F to='user@example.com' \
  -F subject='Password Reset' \
  -F text='Click here to reset your password: https://yourapp.com/reset?token=UNIQUE_TOKEN'

Node.js (Axios):

const axios = require('axios');
const FormData = require('form-data');

const form = new FormData();
form.append('from', 'noreply@yourdomain.com');
form.append('to', 'user@example.com');
form.append('subject', 'Password Reset');
form.append('text', 'Click here to reset your password: https://yourapp.com/reset?token=UNIQUE_TOKEN');

axios.post('https://api.mailgun.net/v3/yourdomain.com/messages', form, {
  headers: {
    ...form.getHeaders(),
    'Authorization': 'Basic ' + Buffer.from('api:YOUR_API_KEY').toString('base64')
  }
})
.then(response => console.log(response.data))
.catch(error => console.error(error));

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Step 5: Handle Responses and Errors

• Success Response: Most APIs return a 200 OK or 202 Accepted status with a message ID (e.g., {"id": "msg_12345"}).
• Error Handling: Check for errors (e.g., invalid API key, unverified domain, or rate limits). Example error response:

  {
    "error": "Invalid API key",
    "code": 401
  }
  

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Step 6: Set Up Webhooks (Optional but Recommended)

Webhooks allow your app to automatically receive notifications for delivery events (e.g., bounces, opens, clicks). Here’s how to set them up:

1. Configure Webhook URL:

   • In your provider’s dashboard, add a webhook endpoint (e.g., https://yourapp.com/webhooks/email).
   • Example (Notify):

     # Webhook payload for a bounce event
     {
       "event": "bounce",
       "email": "user@example.com",
       "reason": "mailbox_full",
       "timestamp": "2026-09-22T12:00:00Z"
     }
     

2. Handle Webhook Events:

   • In your backend, parse the webhook payload and update your app’s logs or user records.
   • Example (Node.js with Express):

     const express = require('express');
     const app = express();
     
     app.post('/webhooks/email', (req, res) => {
       const event = req.body.event;
       const email = req.body.email;
     
       if (event === 'bounce') {
         console.log(`Email bounced for ${email}: ${req.body.reason}`);
         // Update your database or retry logic here
       }
     
       res.status(200).send('OK');
     });
     
     app.listen(3000, () => console.log('Webhook server running on port 3000'));
     

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Step 7: Test Your Setup

1. Send a Test Email:

   • Use the API to send a test email to your own address.
   • Verify it arrives in your inbox (not spam).

2. Check Logs:

   • In your provider’s dashboard, confirm the email was sent and delivered.
   • Example: Notify and Postmark provide delivery logs for debugging.

3. Test Webhooks:

   • Trigger a bounce (e.g., send to a non-existent email) and verify your webhook receives the event.

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Best Practices

1. Use a Verified Domain: Always send from a verified domain (e.g., noreply@yourdomain.com) to improve deliverability and avoid spam filters.
2. Secure Your API Key: Store it in environment variables (e.g., .env file) and never expose it in client-side code.
3. Handle Bounces: Use webhooks to automatically retry or log failed deliveries.
4. Rate Limits: Monitor your provider’s rate limits to avoid throttling.
5. Templates: For dynamic content (e.g., receipts), use HTML templates or a templating engine (e.g., Handlebars, React Email).

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Recommended Providers for Specific Use Cases

Use Case	Recommended Provider	Why?
Simplest setup	Notify	Minimal API, no bloat, built-in logs
Modern developer experience	Resend	Clean API, React Email, TypeScript SDK
Fast delivery	Postmark	Sub-second delivery, detailed logs
Granular control	Mailgun	Inbound/outbound APIs, suppression lists
High deliverability	Mailtrap	Separate streams, analytics
AWS-native teams	Amazon SES	Low-cost, integrates with AWS

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