Missing credentials for «PLAIN»?

let nodemailer = require("nodemailer");

module.exports = function (code) {

	// Use Smtp Protocol to send Email
	let smtpTransport = nodemailer.createTransport({
		host: 'smtp.gmail.com',
		port: 587,
		secure: false,
		auth: {
			user: 'denis@gmail.com',
			password: '228password228'
		}
	});

	let mailOptions = {
		from: "MyAdmin Helper",
		to: "martin1099@mail.ru",
		subject: "Send Email Using Node.js",
		text: `${code}`
	}

	smtpTransport.sendMail(mailOptions, function (error, response) {
		if (error) {
			console.log(error);
		} else {
			console.log("Message sent: " + response.message);
		}

		smtpTransport.close();
	});
}

Пытаюсь отправить код на почту. Ничего не понимаю, зачем нужен auth?? Читал в документации ничего не понятно. Зачем оно надо? В чём проблема?
Выдаёт ошибку :
{ Error: Missing credentials for "PLAIN"
at SMTPConnection._formatError (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:606:19)
at SMTPConnection.login (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:350:38)
at connection.connect (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-transport\index.js:270:32)
at SMTPConnection.once (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:188:17)
at Object.onceWrapper (events.js:273:13)
at SMTPConnection.emit (events.js:182:13)
at SMTPConnection._actionEHLO (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:1128:14)
at SMTPConnection._processResponse (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:762:20)
at SMTPConnection._onData (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:558:14)
at TLSSocket._socket.on.chunk (D:\Programmes\Projects\NodeJs\project\node_modules\nodemailer\lib\smtp-connection\index.js:709:51)
at TLSSocket.emit (events.js:182:13)
at addChunk (_stream_readable.js:283:12)
at readableAddChunk (_stream_readable.js:264:11)
at TLSSocket.Readable.push (_stream_readable.js:219:10)
at TLSWrap.onread (net.js:639:20) code: 'EAUTH', command: 'API' }