node.js mock sendmailer transporter inside function

Im working with an node handler in AWS lambda and i need to make another files with integration tests from that function, but i cant mock the transporter with sinon or mockery.

the handler function:

var nodemailer = require('nodemailer');

exports.handler = (event, context, callback) =>
{
var transporter=createTransporter();
}

function createTransporter() {
return nodemailer.createTransport({
    service: "SMTp",
    auth: {
        user: "XXXX@XXX",
        pass: "XXXX"
    }
});
}

the purpose is to mock the function createTransporter() so that it doesnt send any email when it is called in javascript file test with mocha and expect.

via Daniel