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SDKs

Node.js

Use the official Lettermint Node.js SDK to send email from a Node.js application. The package provides typed requests and responses for the Sending API and Team API. See Integrations for the capability overview.

Requirements

Before you start you need:

• Node.js 20 or newer. The SDK also runs on Bun, Deno, and edge runtimes such as Cloudflare Workers and Vercel Edge, and it has no dependencies.
• A Lettermint account with a verified sending domain
• A Project API token from your project settings

If your domain is not verified yet, follow the domain setup guide first so your mail reaches the inbox instead of the spam folder.

1. Installation

Install the SDK via your preferred package manager:

2. Send your first email

Create the client with your Project API token as the sendingToken:

Code
import { Lettermint } from "lettermint";

const lettermint = new Lettermint({
  sendingToken: process.env.LETTERMINT_PROJECT_TOKEN!,
});

The client stores nothing about a message, so create it once and share it across your app, including across concurrent requests. You can also pass the token as a string, as in new Lettermint(token).

Send your first email:

Code
const response = await lettermint.emails.send({
  from: 'John Doe <john@yourdomain.com>',
  to: ['recipient@example.com'],
  subject: 'Hello from Lettermint',
  text: 'This is a test email sent using the Lettermint Node.js SDK.',
});

console.log(`Email sent with ID: ${response.message_id}`);

emails.send() takes the message in the API's format, for example reply_to and scheduled_at. The to, cc, and bcc fields are arrays.

3. Email features

The examples in this section use lettermint.emails.compose(), a fluent builder. Every setter returns a new builder and leaves the original unchanged, so one builder is safe to share and reuse. Keep the value that a setter returns when you build an email over several statements:

Code
let draft = lettermint.emails
    .compose()
    .from('John Doe <john@yourdomain.com>')
    .to('recipient@example.com')
    .subject('Your invoice');

if (copyAccountant) draft = draft.cc('accountant@yourdomain.com');

await draft.html('<p>Your invoice is ready.</p>').send();

Each builder example is equivalent to a plain object passed to lettermint.emails.send(). Use whichever reads better.

Basic email

Send a simple text or HTML email:

Code
const response = await lettermint.emails
    .compose()
    .from('John Doe <john@yourdomain.com>')
    .to('recipient@example.com')
    .subject('Your account is ready!')
    .html('<h1>Welcome!</h1><p>Thanks for signing up.</p>')
    .text('Welcome! Thanks for signing up.')
    .send();

Multiple recipients

Send to multiple recipients using CC and BCC:

Code
const response = await lettermint.emails
    .compose()
    .from('John Doe <john@yourdomain.com>')
    .to('user1@example.com', 'user2@example.com')
    .cc('manager@yourdomain.com')
    .bcc('archive@yourdomain.com')
    .subject('Monthly Newsletter')
    .html('<h1>This Month\'s Updates</h1>')
    .send();

Custom headers and reply-to

Add custom headers and set reply-to addresses:

Code
const response = await lettermint.emails
    .compose()
    .from('support@yourdomain.com')
    .to('customer@example.com')
    .replyTo('help@yourdomain.com')
    .subject('Support Ticket #12345')
    .headers({
        'X-Priority': '1',
        'X-Ticket-ID': '12345'
    })
    .html('<p>Your support ticket has been updated.</p>')
    .send();

Metadata

Add metadata for tracking and webhook payloads:

Code
const response = await lettermint.emails
    .compose()
    .from('notifications@yourdomain.com')
    .to('user@example.com')
    .subject('Order Confirmation')
    .metadata({
        'order_id': '12345',
        'customer_id': 'cust_789',
        'campaign': 'order_confirmation'
    })
    .html('<p>Your order has been confirmed.</p>')
    .send();

Tags

Categorize emails for filtering and analytics:

Code
const response = await lettermint.emails
    .compose()
    .from('alerts@yourdomain.com')
    .to('admin@example.com')
    .subject('System Alert')
    .tags([
        { name: 'category', value: 'system-alerts' },
        { name: 'environment', value: 'production' }
    ])
    .html('<p>Critical system alert detected.</p>')
    .send();

Route selection

Direct emails to specific routes within your project:

Code
const response = await lettermint.emails
    .compose()
    .from('notifications@yourdomain.com')
    .to('user@example.com')
    .subject('Welcome!')
    .route('transactional')
    .html('<p>Welcome to our platform.</p>')
    .send();

Idempotency

Pass an idempotency key so a retry never sends the same email twice. If a request with the same key is repeated within the idempotency window, Lettermint returns the original result instead of sending again. The key is an option of the call that sends the email:

Code
const response = await lettermint.emails
    .compose()
    .from('notifications@yourdomain.com')
    .to('user@example.com')
    .subject('Order Confirmation')
    .html('<p>Your order has been confirmed.</p>')
    .send({ idempotencyKey: 'order-confirmation-12345' });

emails.send(message, { idempotencyKey }) and emails.sendBatch(messages, { idempotencyKey }) take the same option. The key applies only to that call, and the SDK never retries on its own.

File attachments

Attach files to your emails. content is a base64 string, a Uint8Array (including a Node.js Buffer), or an ArrayBuffer. The SDK encodes bytes to base64 for you:

Code
import { readFile } from 'node:fs/promises';

const response = await lettermint.emails
    .compose()
    .from('invoices@yourdomain.com')
    .to('customer@example.com')
    .subject('Your Invoice')
    .html('<p>Please find your invoice attached.</p>')
    .attach({
        filename: 'invoice.pdf',
        content: await readFile('/path/to/document.pdf'),
        contentType: 'application/pdf'
    })
    .send();

To show an attachment inside the HTML body, set contentId and reference it as cid: in the markup. See Inline images.

Batch sending

Send up to 500 emails in one request. The array can mix plain messages and builders:

Code
const base = lettermint.emails
    .compose()
    .from('John Doe <john@yourdomain.com>')
    .subject('Your weekly summary');

const results = await lettermint.emails.sendBatch([
    { from: 'John Doe <john@yourdomain.com>', to: ['user1@example.com'], subject: 'Hi', text: 'Hello' },
    base.to('user2@example.com').text('Your summary is ready.'),
], { idempotencyKey: 'weekly-summary-2026-10-05' });

4. Send an order confirmation

A real transactional send usually pulls several of these features together. This example sends a receipt with an HTML and plain-text body, tags it for analytics, attaches the order and customer IDs as metadata, and uses an idempotency key so a retry never emails the customer twice:

order-confirmation.ts
import { Lettermint } from "lettermint";

const lettermint = new Lettermint({
  sendingToken: process.env.LETTERMINT_PROJECT_TOKEN!,
});

interface Order {
  id: string;
  number: string;
  total: string;
  customerEmail: string;
  customerId: string;
}

export async function sendOrderConfirmation(order: Order) {
  return await lettermint.emails
    .compose()
    .from('Acme Store <orders@yourdomain.com>')
    .to(order.customerEmail)
    .replyTo('support@yourdomain.com')
    .subject(`Order ${order.number} confirmed`)
    .html(`<h1>Thanks for your order</h1><p>Order ${order.number} totalling ${order.total} is confirmed.</p>`)
    .text(`Thanks for your order. Order ${order.number} totalling ${order.total} is confirmed.`)
    .tags([{ name: 'type', value: 'order-confirmation' }])
    .metadata({ order_id: order.id, customer_id: order.customerId })
    .route('transactional')
    .send({ idempotencyKey: `order-confirmation-${order.id}` });
}

The metadata you attach here travels with every webhook event for the message, so a later delivery or bounce maps straight back to the order in your database.

5. Response

Code
const response = await lettermint.emails
    .compose()
    .from('John Doe <john@yourdomain.com>')
    .to('recipient@example.com')
    .subject('Test')
    .text('Hello!')
    .send();

console.log(`Email sent with ID: ${response.message_id}`);
console.log(`Status: ${response.status}`);

6. Handle errors

Every error the SDK throws extends LettermintError. API errors extend ApiError, which carries status, code, and message. Catch the subclasses you want to handle:

Code
import { ApiError, RateLimitError, TimeoutError, ValidationError } from "lettermint";

try {
  await lettermint.emails.send(message, { idempotencyKey });
} catch (error) {
  if (error instanceof ValidationError) {
    console.error(error.message, error.errors);
  } else if (error instanceof RateLimitError) {
    // Wait error.retryAfter seconds, then retry with the same idempotencyKey.
  } else if (error instanceof TimeoutError) {
    // The outcome is unknown. Retry with the same idempotencyKey.
  } else if (error instanceof ApiError) {
    console.error(error.status, error.code, error.message);
  } else {
    throw error;
  }
}

The SDK never retries on its own. Reuse the same idempotency key when you retry so the customer gets one email. Other errors include AuthenticationError, PermissionError, NotFoundError, ConflictError, ServerError, ConnectionError, LettermintConfigError for a missing token, and LettermintValidationError when the SDK rejects a request before it sends it.

Track delivery, opens, and bounces

Sending is only half of a transactional setup. To see what happens after a message leaves your app, combine the SDK with Lettermint's platform features:

• Email tracking records opens and clicks, with bot filtering so your metrics stay accurate.
• Webhooks push delivery, bounce, and complaint events to your server in real time, so you can update order records or suppress bad addresses. The SDK verifies each delivery with new Webhook(secret).verify(rawBody, headers). See Webhook signatures.
• Test addresses let you trigger a hard or soft bounce on demand while you build and verify your webhook handler.

Tracking is configured per route, so turn it on for the route your app sends through, then read the results from your dashboard or your webhook endpoint.

FAQ

Does the SDK support TypeScript?

Yes. The SDK is written in TypeScript and exports its request and response types, such as SendMailRequest and SendMailResponse, so every builder method and field is typed and autocompletes in your editor. Query parameters for the Team API are typed objects too, for example { page: { size: 30 } }.

How do I stop the same email being sent twice?

Pass an idempotencyKey option to send() or sendBatch(), derived from a stable ID such as an order number. A repeated request with the same key returns the original result instead of sending again. See the idempotency documentation.

Is Lettermint email GDPR compliant and EU-hosted?

Yes. Lettermint runs exclusively on European infrastructure and processes email in line with GDPR, so transactional mail from your Node app is handled inside the EU.

How do I track opens, clicks, and bounces?

Enable tracking on your route and subscribe to webhooks to receive delivery, open, and bounce events. Use the test addresses to simulate bounces while you build your handler.

Next steps