mailcomposer is a Node.JS module for generating e-mail messages that can be streamed to SMTP or file.

This is a standalone module that only generates raw e-mail source, you need to write your own or use an existing transport mechanism (SMTP client, Amazon SES, SendGrid etc). mailcomposer frees you from the tedious task of generating rfc2822 compatible messages.

Build Status

mailcomposer supports:

  • Unicode to use any characters ✔
  • HTML content as well as plain text alternative
  • Attachments and streaming for larger files (use strings, buffers, files or binary streams as attachments)
  • Embedded images in HTML
  • DKIM signing
  • usage of your own transport mechanism

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Install through NPM

npm install mailcomposer


Include mailcomposer module

var MailComposer = require("mailcomposer").MailComposer;

Create a new MailComposer instance

var mailcomposer = new MailComposer([options]);

Where options is an optional options object with the following possible properties:

  • escapeSMTP - if set replaces dots in the beginning of a line with double dots
  • encoding - sets transfer encoding for the textual parts (defaults to "quoted-printable")
  • charset - sets output character set for strings (defaults to "utf-8")
  • keepBcc - if set to true, includes Bcc: field in the message headers. Useful for sendmail command.
  • forceEmbeddedImages - convert image urls and absolute paths in HTML to embedded attachments.

Simple example

The following example generates a simple e-mail message with plaintext and html body.

var MailComposer = require("mailcomposer").MailComposer;
    mailcomposer = new MailComposer(),
    fs = require("fs");

// add additional header field
mailcomposer.addHeader("x-mailer", "Nodemailer 1.0");

// setup message data
    from: "",
    to: "",
    body: "Hello world!",
    html: "<b>Hello world!</b>"


// pipe the output to a file

The output for such a script (the contents for "test.eml") would look like:

MIME-Version: 1.0
X-Mailer: Nodemailer 1.0
Content-Type: multipart/alternative;

Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Hello world!
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

<b>Hello world!</b>


Add custom headers

Headers can be added with mailcomposer.addHeader(key, value[, formatted]) where formatted indicates if the value should be kept as is. If the value is missing or falsy, header value is sanitized and folded. If true, the value is passed to output as is.

var mailcomposer = new MailComposer();
mailcomposer.addHeader("x-mailer", "Nodemailer 1.0");

If you add an header value with the same key several times, all of the values will be used in the generated header. For example:

mailcomposer.addHeader("x-mailer", "Nodemailer 1.0");
mailcomposer.addHeader("x-mailer", "Nodemailer 2.0");

Will be generated into

X-Mailer: Nodemailer 1.0
X-Mailer: Nodemailer 2.0

The contents of the field value is not edited in any way (except for the folding), so if you want to use unicode symbols you need to escape these to mime words by yourself. Exception being object values - in this case the object is automatically JSONized and mime encoded.

// using objects as header values is allowed (will be converted to JSON)
var apiOptions = {};
apiOptions.category = "newuser";
apiOptions.tags = ["user", "web"];
mailcomposer.addHeader("X-SMTPAPI", apiOptions)

Add message parts

You can set message sender, receiver, subject line, message body etc. with mailcomposer.setMessageOption(options) where options is an object with the data to be set. This function overwrites any previously set values with the same key

The following example creates a simple e-mail with sender being, receiver and plaintext part of the message as Hello world!:

    from: "",
    to: "",
    body: "Hello world!"

Possible options that can be used are (all fields accept unicode):

  • from (alias sender) - the sender of the message. If several addresses are given, only the first one will be used
  • to - receivers for the To: field
  • cc - receivers for the Cc: field
  • bcc - receivers for the Bcc: field
  • replyTo (alias reply_to) - e-mail address for the Reply-To: field
  • inReplyTo - The message-id this message is replying
  • references - Message-id list
  • subject - the subject line of the message
  • body (alias text) - the plaintext part of the message
  • html - the HTML part of the message
  • envelope - optional SMTP envelope, if auto generated envelope is not suitable

This method can be called several times

mailcomposer.setMessageOption({from: ""});
mailcomposer.setMessageOption({to: ""});
mailcomposer.setMessageOption({body: "Hello world!"});

Trying to set the same key several times will yield in overwrite

mailcomposer.setMessageOption({body: "Hello world!"});
mailcomposer.setMessageOption({body: "Hello world?"});
// body contents will be "Hello world?"

Address format

All e-mail address fields take structured e-mail lists (comma separated) as the input. Unicode is allowed for all the parts (receiver name, e-mail username and domain) of the address. If the domain part contains unicode symbols, it is automatically converted into punycode, user part will be converted into UTF-8 mime word.

E-mail addresses can be a plain e-mail addresses

or with a formatted name

'Ноде Майлер' <>

Or in case of comma separated lists, the formatting can be mixed, 'Ноде Майлер' <>, "Name, User" <>

SMTP envelope

SMTP envelope is usually auto generated from from, to, cc and bcc fields but if for some reason you want to specify it yourself, you can do it with envelope property.

envelope is an object with the following params: from, to, cc and bcc just like with regular mail options. You can also use the regular address format.

mailOptions = {
    from: "",
    to: "",
    envelope: {
        from: "Daemon <>",
        to: ", Mailer <>"

Add attachments

Attachments can be added with mailcomposer.addAttachment(attachment) where attachment is an object with attachment (meta)data with the following possible properties:

  • fileName (alias filename) - filename to be reported as the name of the attached file, use of unicode is allowed
  • cid - content id for using inline images in HTML message source
  • contents - String or a Buffer contents for the attachment
  • filePath - path to a file or an URL if you want to stream the file instead of including it (better for larger attachments)
  • streamSource - Stream object for arbitrary binary streams if you want to stream the contents (needs to support pause/resume)
  • contentType - content type for the attachment, if not set will be derived from the fileName property
  • contentDisposition - content disposition type for the attachment, defaults to "attachment"
  • userAgent - User-Agent string to be used if the fileName points to an URL

One of contents, filePath or streamSource must be specified, if none is present, the attachment will be discarded. Other fields are optional.

Attachments can be added as many as you want.

Using embedded images in HTML

Attachments can be used as embedded images in the HTML body. To use this feature, you need to set additional property of the attachment - cid (unique identifier of the file) which is a reference to the attachment file. The same cid value must be used as the image URL in HTML (using cid: as the URL protocol, see example below).

NB! the cid value should be as unique as possible!

var cid_value = + '.image.jpg';

var html = 'Embedded image: <img src="cid:' + cid_value + '" />';

var attachment = {
    fileName: "image.png",
    filePath: "/static/images/image.png",
    cid: cid_value

Automatic embedding images

If you want to convert images in the HTML to embedded images automatically, you can set mailcomposer option forceEmbeddedImages to true. In this case all images in the HTML that are either using an absolute URL (http://...) or absolute file path (/path/to/file) are replaced with embedded attachments.

For example when using this code

var mailcomposer = new MailComposer({forceEmbeddedImages: true});
    html: 'Embedded image: <img src=""&gt;'

The image linked is fetched and added automatically as an attachment and the url in the HTML is replaced automatically with a proper cid: string.

Add alternatives to HTML and text

In addition to text and HTML, any kind of data can be inserted as an alternative content of the main body - for example a word processing document with the same text as in the HTML field. It is the job of the e-mail client to select and show the best fitting alternative to the reader.

Alternatives to text and HTML can be added with mailcomposer.addAlternative(alternative) where alternative is an object with alternative (meta)data with the following possible properties:

  • contents - String or a Buffer contents for the attachment
  • contentType - optional content type for the attachment, if not set will be set to "application/octet-stream"
  • contentEncoding - optional value of how the data is encoded, defaults to "base64"

If contents is empty, the alternative will be discarded. Other fields are optional.

Usage example:

// add HTML "alternative"
    html: "<b>Hello world!</b>"

// add Markdown alternative
    contentType: "text/x-web-markdown",
    contents: "Hello world!"

If the receiving e-mail client can render messages in Markdown syntax as well, it could prefer to display this alternative as the main content of the message.

Alternatives can be added as many as you want.

DKIM Signing

mailcomposer supports DKIM signing with very simple setup. Use this with caution though since the generated message needs to be buffered entirely before it can be signed - in this case the streaming capability offered by mailcomposer is illusionary, there will only be one 'data' event with the entire message. Not a big deal with small messages but might consume a lot of RAM when using larger attachments.

Set up the DKIM signing with useDKIM method:


Where dkimOptions includes necessary options for signing

  • domainName - the domainname that is being used for signing
  • keySelector - key selector. If you have set up a TXT record with DKIM public key at then zzz is the selector
  • privateKey - DKIM private key that is used for signing as a string
  • headerFieldNames - optional colon separated list of header fields to sign, by default all fields suggested by RFC4871 #5.5 are used

NB! Currently if several header fields with the same name exists, only the last one (the one in the bottom) is signed.


mailcomposer.setMessageOption({from: ""});
mailcomposer.setMessageOption({to: ""});
mailcomposer.setMessageOption({body: "Hello world!"});
    domainName: "",
    keySelector: "dkim",
    privateKey: fs.readFileSync("private_key.pem")

Start streaming

When the message data is setup, streaming can be started. After this it is not possible to add headers, attachments or change body contents.


This generates 'data' events for the message headers and body and final 'end' event. As MailComposer objects are Stream instances, these can be piped

// save the output to a file

Compile the message in one go

If you do not want to use the streaming possibilities, you can compile the entire message into a string in one go with buildMessage.

mailcomposer.buildMessage(function(err, messageSource){
    console.log(err || messageSource);

The function is actually just a wrapper around streamMessage and emitted events.


Envelope can be generated with an getEnvelope() which returns an object that includes a from address (string) and a list of to addresses (array of strings) suitable for forwarding to a SMTP server as MAIL FROM: and RCPT TO:.

// {from:"", to:[""]}

NB! both from and to properties might be missing from the envelope object if corresponding addresses were not detected from the e-mail.

Running tests

Tests are run with nodeunit


npm test