Stuck in a express callback hell - node.js

Question

I can't cancel the request after sending the email successfully. How can I return properly after sending the email? This might be a callback hell, but I cant figure out how to solve it.
I tried to put some return in different parts but it didn't work.
const router = require('express').Router();
const nodemailer = require('nodemailer');
const emailExistence= require('email-existence');
module.exports = router;
// Send email when user has forgotten his/her password
router.post('/forgetPass', (req, res, next) => {
if(!req.body.email){
next(new Error("Email is required."));
return;
}
emailExistence.check(req.body.email, function(err,res){
if(err || !res){
next(new Error("The email does'nt exist."));
return;
}else{
let transporter = nodemailer.createTransport({
service: 'gmail',
auth: {
user: 'myemail#gmail.com',
pass: 'mypassword'
}
});
let mailOptions = {
from: 'myemail#gmail.com',
to: req.body.email,
subject: 'Link for setting a new password',
html: 'Set a new password'
text: 'email text'
};
transporter.sendMail(mailOptions, function(error, info){
if (error) {
next(new Error("Error in sending email."));
return;
}
res.json(Object.assign(req.base, {
message: "The email has been sent successfully.",
data: info
}));
return;
});
}
});
});

Answer 1

Once you set your response field on successful sending, call next() as a last step, so the next middleware gets the request and sends the response back. So basically:
...
res.json(yourResponse);
next();
...
Or, if this is the last middleware, send the response back to client:
res.send(yourResponse);

Answer 2

I solved it in this way. The emailExistence didn't let me to use promises, so I used email-ckeck instead of it:
const router = require('express').Router();
const nodemailer = require('nodemailer');
const emailExistence= require('email-existence');
var emailCheck = require('email-check');
module.exports = router;
router.post('/forgetPass', (req, res, next) => {
if(!req.body.email){
next(new Error("Email is required."));
return;
}
// Check the req.body.email with email pattern regex
var patt = new RegExp (process.env.EMAIL_PATTERN__REGEX),
isEmail = patt.test(req.body.email);
if(!isEmail){
next(new Error("The email does'nt seem to be a valid email. If you are sure about your email validity contact the website admin."));
return;
}
return emailCheck(req.body.email)
.then(function(result){
let transporter = nodemailer.createTransport({
service: 'gmail',
auth: {
user: process.env.EMAIL,
pass: process.env.EMAILPASSWORD
}
});
let mailOptions = {
from: process.env.EMAIL,
to: req.body.email,
subject: 'Link for setting a new password',
html: 'Set a new password from this link.'
};
return transporter.sendMail(mailOptions)
.then(function (result2) {
res.status(200).json(Object.assign(req.base, {
message: "The email has been sent successfully.",
data: null
}));
return;
},
function(error2){
next(new Error("Error in sending email."));
return;
});
},
function(error) {
next(new Error("The email does'nt seem to be a valid email. If you are sure about your email validity contact the website admin."));
return;
});
});