Ejemplo uso de mailer en express app

app-con-mailer-ejemplo.js

const mailer = require('express-mailer');
//..
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
//...
const GMAIL_USER = process.env.GMAIL_USER;
const GMAIL_PASSWORD = process.env.GMAIL_PASSWORD;

mailer.extend(app, {
    from: 'no-reply@aviva.pe',
    host: 'smtp.gmail.com', // hostname
    secureConnection: true, // use SSL
    port: 465, // port for secure SMTP
    transportMethod: 'SMTP', // default is SMTP. Accepts anything that nodemailer accepts
    auth: {
        user: GMAIL_USER,
        pass: GMAIL_PASSWORD
    }
});

//...
app.post('/test', (req, res) => {
    const mailerData = {
            to: req.body.to,
            bcc: req.body.bcc,
            subject: req.body.subject,
            extra: 'texto libre',
            body: req.body.message,
        };

    // usar vista views/mail
    mailerSend('./views/mail', mailerData).then((mailerResult) => {
        return res.json({ mailerData, mailerResult });
    });
});