Stub transport is useful for testing the email before actually sending email. It does not actually send an email, it convert mail object to a single buffer and return it with a send mail callback.

In the below code, I have stubbed send method of nodemailer, using sinon module. Please check the below code:

const sinon = require('sinon');
const nodemailer = require('nodemailer');

describe('Forgot password', () => {
    const middleware = server(0, true);
    const baseUrl = '/api/v1';
    let nm = '';
    let transport = '';

    beforeEach(cb => {
        transport = {
            name: 'testsend',
            version: '1',
            send: function send(data, callback) {
                callback();
            },
            logger: false,
        };
        nm = nodemailer.createTransport(transport);
        cb();
    });

    context('when user exist in database', () => {
        it('repond with 200 request with correct code that exist', function test(done) {
            this.timeout(15000);

            sinon.stub(transport, 'send').yields(null, 'test');

            nm.sendMail({
                subject: 'test',
            }, (err, info) => {
                transport.send.restore();
                done();
            });

            request(middleware)
                .post(`${baseUrl}/recover-password`)
                .send({
                    code: 'sss'
                })
                .set('Accept', 'application/json')
                .expect('Content-Type', /json/)
                .expect(httpConstants.STATUS_OK_200, {
                    message: 'Successfully done'
                })
                .end(done);
        });
    });
});

We have an alternative for the above code, that is using nodemailer-stub-transport module, but for that you have to modify your server side code, the code is as follows:
 

var nodemailer = require('nodemailer');

var stubTransport = require('nodemailer-stub-transport');


var transport;
if (/* are you testing */) {
    transport = nodemailer.createTransport(stubTransport());  
} else {
    transport = nodemailer.createTransport(/* your normal config */);
 }


module.exports = transport;

You can set the mode (If it is in development or testing) using process global object as below:
 

process.env.NODE_ENV= 'mocha_test';