[Audaxviator]

Рас

function foo(email, password) {

	return new Promise(function(resolve, reject) {

		nodemailer.createTestAccount(function(err, account) {
			var transporter = nodemailer.createTransport({
				//service: 'Gmail',
				host: 'smtp.ethereal.email',
				port: 587,
				secure: true, // use SSL
				auth: {
					user: account.user,
					pass: account.pass
				}
			});

			var message = {
				from: '"Heano" <games.1212@yandex.ru>',
				to: email,
				subject: 'Регистрация ✔',
				text: 'Твой пароль: '+ password,
				html: 'Твой пароль:<b> '+ password+'</b>'
			};

			transporter.sendMail(message, function (err, info) {
					if (err) {
							resolve('Error:', err.message);
							return;
					}
					resolve('Server responded with ' + info.response);
			});
		});
	});
}

Два

foo('f@ya.ru', '1')
	.then(function(info) {
		console.log(info);
	})
	.catch(function(err) {
		console.log(err);
	});