"code": "ECONNECTION", "command": "CONN"

https://stackoverflow.com/questions/38431592/sending-mail-in-node-js-using-nodemailer

La solución al problema que presenté arriba se resuelve en 2 pasos:

1. agregar en el código:

antes del transporter:   process.env.NODE_TLS_REJECT_UNAUTHORIZED = "0";

  process.env.NODE_TLS_REJECT_UNAUTHORIZED = "0";
 
    // create reusable transporter object using the default SMTP transport
    let transporter = nodemailer.createTransport({
        host: 'smtp.gmail.com',
        port: 465,
        secure: true,

2. https://myaccount.google.com/lesssecureapps?pli=1

Habilitar Allow less secure apps: ON, en gmail.