Stream transport

Stream transport is not a real SMTP transport. Instead of handing the message off to a remote mail server it builds the complete RFC 822 e‑mail and gives it back to you. This makes it perfect for

• Testing – inspect the exact bytes that would be sent over the wire, run snapshot tests, or feed the output to another system.
• Custom delivery pipelines – run Nodemailer plugins (DKIM, list headers, etc.) and then deliver the message yourself via an in‑house API, store it for audit logging, and so on.

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Enabling Stream transport

Create the transporter just like any other Nodemailer transport, but pass streamTransport: true in the constructor options:

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  streamTransport: true,
  // optional tweaks shown below
});

Options

Option	Type	Default	Description
streamTransport	boolean	required	Enable Stream transport.
buffer	boolean	false	Return the generated message as a Buffer instead of a Readable stream.
newline	'windows' | 'unix'	'unix'	New‑line format for the generated message: CR LF (\r\n) for Windows or LF (\n) for Unix‑style.
jsonTransport	boolean	false	Alternative to streamTransport. Returns a JSON string representation of the message (see below).

sendMail() callback signature

The callback receives (err, info) where info contains:

• envelope – the SMTP envelope { from, to }.
• messageId – the Message‑ID header value.
• message – a Node.js Readable stream (default) or a Buffer/JSON string depending on the options you set.

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Examples

1. Stream a message with Windows‑style new‑lines

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  streamTransport: true,
  newline: "windows", // CRLF (\r\n)
});

transporter.sendMail(
  {
    from: "sender@example.com",
    to: "recipient@example.com",
    subject: "Streamed message",
    text: "This message is streamed using CRLF new‑lines.",
  },
  (err, info) => {
    if (err) throw err;
    console.log(info.envelope);
    console.log(info.messageId);
    // Pipe the raw RFC 822 message to STDOUT
    info.message.pipe(process.stdout);
  }
);

2. Return a Buffer with Unix‑style new‑lines

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  streamTransport: true,
  buffer: true, // return Buffer instead of Stream
  newline: "unix", // LF (\n)
});

transporter.sendMail(
  {
    from: "sender@example.com",
    to: "recipient@example.com",
    subject: "Buffered message",
    text: "This message is buffered using LF new‑lines.",
  },
  (err, info) => {
    if (err) throw err;
    console.log(info.envelope);
    console.log(info.messageId);
    // The complete message is in a single Buffer
    console.log(info.message.toString());
  }
);

3. Generate a JSON‑encoded message object (≥ v3.1.0)

Pass jsonTransport: true (and omit streamTransport). The resulting info.message is a serialized JSON string that you can later feed back to Nodemailer or inspect in your tests. Binary data such as attachments is base64‑encoded.

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  jsonTransport: true,
});

transporter.sendMail(
  {
    from: "sender@example.com",
    to: "recipient@example.com",
    subject: "JSON message",
    text: "I hope this message gets JSON‑ified!",
  },
  (err, info) => {
    if (err) throw err;
    console.log(info.envelope);
    console.log(info.messageId);
    console.log(info.message); // JSON string
  }
);

An abbreviated example of the JSON payload:

{
  "from": { "address": "sender@example.com", "name": "" },
  "to": [{ "address": "recipient@example.com", "name": "" }],
  "subject": "JSON message",
  "text": "I hope this message gets JSON‑ified!",
  "headers": {},
  "messageId": "<77a3458f-8070-339d-095f-85bb73f3db8e@example.com>"
}

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When to choose Stream vs. JSON transport

Use case	Recommended option
Inspect or pipe raw SMTP content	streamTransport (Stream or Buffer)
Persist structured message for later replay	jsonTransport
Need to support Nodemailer plugins	Both (plugins run before serialization)
Want _raw property support	Stream transport only