Using simple-parse-smtp-adapter with cloud code, Parse Server, https://github.com/lcortess/simple-parse-smtp-adapter

main.js

var sendSmtpMail = require('simple-parse-smtp-adapter')({
    fromAddress: '[email protected]',
    user: '[email protected]',
    password: 'AwesomePassword',
    host: 'your.smtp.host',
    isSSL: true,
    port: 465,
    emailField: 'email'
}).sendMail;

Parse.Cloud.afterSave("_User", function (request) {
    sendSmtpMail({
        to: request.get('email'),
        text: 'hi,<br> Message!',
        subject: 'subject'
    });
});