index.js

var nodemailer = require('nodemailer')

var sendOptions
var sendCallback
var shouldFail = false
var suppliedCallback
var triggered = false
var response = {
  message: 'Caught by mockmailer',
  messageId: 'noid'
}
var error = new Error('failed by user request')

nodemailer.createTransport = function () {
  return {
    sendMail: function (opts, cb) {
      sendOptions = opts
      sendCallback = cb
      triggered = true
      delete response.options
      delete response.sendCallback

      if (typeof cb === 'function') {
        cb(shouldFail ? error : null, response)
      }

      if (typeof suppliedCallback === 'function') {
        response.options = sendOptions
        response.callback = sendCallback
        suppliedCallback(shouldFail ? error : null, response)
        triggered = false
      }
    }
  }
}

/**
 * Supply a callback to be invoked when mail is sent that resolves or fails your test
 * @method  mockmailer
 * @async
 * @param   {function} registeredCallback The callback to invoke when `sendMail` is called
 * @returns {object} undefined
 */
var mockmailer = module.exports = function (registeredCallback) {
  if (typeof registeredCallback === 'function') {
    suppliedCallback = registeredCallback
    if (triggered) {
      response.options = sendOptions
      response.callback = sendCallback
      suppliedCallback(shouldFail ? error : null, response)
      triggered = false
    }
  }
}

/**
 * Sets nodemailer to either fail or not when `sendMail` is called
 * @method setFail
 * @param  {boolean} mode Should nodemailer "fail" sending mail
 */
mockmailer.setFail = function (mode) {
  shouldFail = !!mode
}