How to Send Email with Node.js

Method 1: REST API with fetch

Node.js 18+ includes the built-in fetch API. No dependencies required — just a single HTTP POST request.

const response = await fetch("https://api.relaypost.dev/v1/emails/send", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "Authorization": "Bearer YOUR_API_KEY",
  },
  body: JSON.stringify({
    from: { email: "[email protected]", name: "Your App" },
    to: [{ email: "[email protected]" }],
    subject: "Your order has shipped",
    html: "<h1>Order Shipped</h1><p>Your package is on the way.</p>",
  }),
});

const result = await response.json();
console.log(result.data.message_id);

This works in any Node.js 18+ environment including serverless functions (AWS Lambda, Vercel, Cloudflare Workers).

Method 2: SMTP with Nodemailer

If your framework expects SMTP or you want to switch providers without code changes, use Nodemailer.

// npm install nodemailer
const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  host: "smtp.relaypost.dev",
  port: 587,
  secure: false, // STARTTLS
  auth: {
    user: process.env.SMTP_USERNAME,
    pass: process.env.SMTP_PASSWORD,
  },
});

await transporter.sendMail({
  from: '"Your App" <[email protected]>',
  to: "[email protected]",
  subject: "Your order has shipped",
  html: "<h1>Order Shipped</h1><p>Your package is on the way.</p>",
});

Error handling

Always handle errors when sending email. Common failures include invalid API keys, rate limiting, and validation errors.

try {
  const response = await fetch("https://api.relaypost.dev/v1/emails/send", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Authorization": "Bearer YOUR_API_KEY",
    },
    body: JSON.stringify({
      from: { email: "[email protected]" },
      to: [{ email: "[email protected]" }],
      subject: "Hello",
      html: "<p>Hello from RelayPost</p>",
    }),
  });

  if (!response.ok) {
    const error = await response.json();
    console.error("Send failed:", error.error.code, error.error.message);
  }
} catch (err) {
  console.error("Network error:", err.message);
}

Sending with attachments

Include file attachments by base64-encoding the content and adding it to the attachments array.

const fs = require("fs");

const response = await fetch("https://api.relaypost.dev/v1/emails/send", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "Authorization": "Bearer YOUR_API_KEY",
  },
  body: JSON.stringify({
    from: { email: "[email protected]" },
    to: [{ email: "[email protected]" }],
    subject: "Your invoice",
    html: "<p>Please find your invoice attached.</p>",
    attachments: [{
      filename: "invoice.pdf",
      content: fs.readFileSync("./invoice.pdf").toString("base64"),
      type: "application/pdf",
    }],
  }),
});

Express.js example

A complete Express route that sends a welcome email after user registration.

const express = require("express");
const app = express();
app.use(express.json());

app.post("/register", async (req, res) => {
  // ... create user in database ...

  await fetch("https://api.relaypost.dev/v1/emails/send", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Authorization": "Bearer " + process.env.RELAYPOST_API_KEY,
    },
    body: JSON.stringify({
      from: { email: "[email protected]", name: "YourApp" },
      to: [{ email: req.body.email }],
      subject: "Welcome to YourApp",
      html: "<h1>Welcome!</h1><p>Your account is ready.</p>",
    }),
  });

  res.json({ success: true });
});

app.listen(3000);

Best practices

• Store API keys in environment variables, never in source code
• Set up SPF and DKIM for your sending domain before sending production email
• Include both html and text versions for maximum compatibility
• Handle errors gracefully — queue failed sends for retry
• Use webhooks to track delivery status instead of polling
• Keep email sending out of the request path — use a background job queue for non-critical emails