Skip to main content

Delivering bulk mail

Sending at “newsletter scale” (hundreds of thousands to tens of millions of messages) is a very different problem from sending account‑sign‑up confirmations. This page collects practical advice that has worked well for teams sending 10 million+ messages with Nodemailer.

1. Use an infrastructure that is made for bulk​

Choose a specialist email service provider (ESP). Free or “included” SMTP servers usually throttle or block you after a few hundred messages.

2. Pull messages from a queue – don’t push everything into memory​

Store work in a durable queue (RabbitMQ, Amazon SQS, Redis Streams, etc.). Start N workers that each:

  1. Wait until the transporter has a free slot.
  2. Pop the next job from the queue.
  3. Call sendMail().
  4. Acknowledge the job only after the callback resolves – failed deliveries are retried automatically.

3. Reuse connections with pooled SMTP​

const nodemailer = require("nodemailer");

const transport = nodemailer.createTransport({
  host: "smtp.example.com",
  port: 587,
  secure: false, // STARTTLS
  auth: {
    user: process.env.SMTP_USER,
    pass: process.env.SMTP_PASS,
  },

  /* Pool options */
  pool: true, // enable pooled SMTP
  maxConnections: 20, // tune based on provider limits and your CPU
  maxMessages: Infinity, // keep the connection open
  // Optional but useful:
  rateDelta: 1000, // window for rateLimit (1 s)
  rateLimit: 100, // max 100 msgs per rateDelta
});

Why pooled SMTP?​

  • One TCP/TLS handshake per hundreds of messages instead of per message.
  • Dramatically fewer authentication round‑trips.
  • Warm connections keep you under connection‑rate limits many ESPs enforce.

4. Tune the pool size​

maxConnections is the main lever. Start small (10–20) and monitor:

  • CPU load and memory usage on the worker machine.
  • The ESP’s connections and messages‑per‑second metrics.
  • SMTP 421 rate‑limit responses – if you see these, back off.

5. Keep attachments on disk​

If every recipient receives the same PDF, point Nodemailer to the file path instead of an HTTP/HTTPS URL:

attachments: [
  {
    filename: "catalogue.pdf",
    path: "/srv/bulk/attachments/catalogue.pdf",
  },
];

Most OSes cache hot files aggressively — a disk read is far cheaper than 10 million HTTP requests to your own server.

6. Prefer bulk‑aware HTTP APIs – if your ESP offers them​

An HTTP “send‑to‑many” endpoint can outperform SMTP because the ESP expands the template server‑side – one API call instead of millions of SMTP transactions. Where only single‑recipient HTTP endpoints are available, stick with pooled SMTP.

Next – monitoring and retries​

  • Track per‑recipient SMTP response codes.
  • Exponential back‑off for 4xx soft‑bounces.
  • Periodic re‑queueing of transient failures.

Nodemailer emits the full SMTP response in info.response – persist this with each job so you can inspect and retry intelligently.