Stream transport

Stream transport is not a real SMTP transport. Instead of delivering your message to a remote mail server, it generates the complete RFC 822 formatted email and returns it to you. This makes it ideal for:

• Testing - Examine the exact bytes that would be sent over the wire, create snapshot tests, or forward the output to another system for validation.
• Custom delivery pipelines - Apply Nodemailer plugins (such as DKIM signing or list headers) to your message, then handle delivery yourself through an internal API, archive messages for audit logging, or process them in any custom way.

For an overview of all available transports, see the transports documentation.

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Enabling Stream transport

To use Stream transport, create a transporter with streamTransport: true in the options object:

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  streamTransport: true,
  // See additional options below
});

Options

Option	Type	Default	Description
streamTransport	boolean	required	Set to true to enable Stream transport.
buffer	boolean	false	When true, returns the generated message as a Buffer instead of a Readable stream.
newline	'windows' | 'unix'	'unix'	Line ending style for the generated message. Use 'windows' for CRLF (\r\n) or 'unix' for LF (\n).

JSON Transport

A separate JSON transport is also available. Enable it by setting jsonTransport: true (instead of streamTransport). JSON transport returns a serialized JSON representation of the message rather than the raw RFC 822 format. See the JSON transport section below for details.

sendMail() callback signature

The sendMail() callback receives two arguments: (err, info). On success, the info object contains:

• envelope - The SMTP envelope object with from (string) and to (array of strings) properties.
• messageId - The generated Message-ID header value for this email.
• message - The generated email content. By default this is a Node.js Readable stream. If you set buffer: true, it will be a Buffer. For JSON transport, it will be a JSON string (or a plain object if skipEncoding: true).

For details on configuring the message object passed to sendMail(), see the message configuration documentation. To parse the generated RFC 822 stream output, you can use MailParser.

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Examples

1. Stream a message with Windows-style line endings

This example generates an email as a readable stream using Windows-style CRLF line endings. The stream can be piped to any writable destination.

const nodemailer = require("nodemailer");

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

transporter.sendMail(
  {
    from: "sender@example.com",
    to: "recipient@example.com",
    subject: "Streamed message",
    text: "This message is streamed using CRLF line endings.",
  },
  (err, info) => {
    if (err) throw err;
    console.log(info.envelope);   // { from: '...', to: ['...'] }
    console.log(info.messageId);  // '<unique-id@example.com>'
    // Pipe the raw RFC 822 message to stdout
    info.message.pipe(process.stdout);
  }
);

2. Return a Buffer with Unix-style line endings

When you need the entire message in memory at once, set buffer: true. This example also explicitly uses Unix-style LF line endings (the default).

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  streamTransport: true,
  buffer: true,    // Return a Buffer instead of a stream
  newline: "unix", // Use LF (\n) line endings (this is the default)
});

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

3. Generate a JSON-encoded message object (>= v3.1.0)

JSON transport is a separate transport type, not an option of Stream transport. To use it, set jsonTransport: true instead of streamTransport. The resulting info.message will be a JSON string representing the message structure. This format is useful for storing messages, inspecting them in tests, or passing them to other systems. Binary data such as attachments is automatically base64-encoded.

If you prefer to work with a JavaScript object rather than a JSON string, set skipEncoding: true to receive the raw data object directly.

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
  }
);

Here is an example of what the JSON output looks like:

{
  "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 the following table to help decide which transport best fits your needs:

Use case	Recommended transport
Inspect or pipe raw RFC 822 SMTP content	streamTransport (Stream or Buffer)
Store structured message data for later replay	jsonTransport
Apply Nodemailer plugins (DKIM, headers, etc.)	Either (plugins run before output)
Need access to the _raw property (see custom source)	Stream transport only