Check DNS MX entries to validate if the domain part of a given email is valid.

email-mx-check.js

const dns = require('dns');

/**
 * Mail Exchange (MX) records are DNS records that are necessary for delivering email to your address.
 * In simple DNS terms, an MX record is used to tell the world which mail servers accept incoming mail 
 * for a given domain. So we can use the domain part of any email to check MX records, if there is
 * records it means its a valid email service, otherwise is not.
 * 
 * WARNING: By checking the MX Records you dont check if the email is valid or if exists, you check
 * if the EMAIL SERVICE is valid and exists. Anyway is a good filter for spammers and avoid waste
 * resources with obviously invalid emails.
 * 
 * This functions accepts and email and returns a Promise with the MX records, meaning its a valid
 * email service, or an error, meaning that the MX doenst exists or the checking has failed.
 * 
 * @param {String} email 
 * @returns {Promise}
 */
function checkMX(email) {
  const domain = email.split('@')[1];

  return new Promise((resolve, reject) => {
    dns.resolve(domain, 'MX', (err, mxAddrs) => {
      err && reject(err);
      
      mxAddrs && mxAddrs.length ? 
        resolve(mxAddrs) :
        reject(new Error(`No MX entries found for "${domain}"`));
    });		
  });
}

/** 
 * Usage: 
 * 
 * For a valid email, will return something like:
 * 
 * [ { exchange: 'alt1.gmail-smtp-in.l.google.com', priority: 10 },
 *  { exchange: 'alt2.gmail-smtp-in.l.google.com', priority: 20 },
 *  { exchange: 'alt4.gmail-smtp-in.l.google.com', priority: 40 },
 *  { exchange: 'gmail-smtp-in.l.google.com', priority: 5 },
 *  { exchange: 'alt3.gmail-smtp-in.l.google.com', priority: 30 } ]
 */

checkMX('[email protected]')
  .then(console.log)
  .catch(console.error);

/**
 * Usage:
 * 
 * For and invalid email, will return something like
 * 
 * { Error: queryMx ENOTFOUND whatever123a.com
 *     at QueryReqWrap.onresolve [as oncomplete] (dns.js:196:19)
 *   errno: 'ENOTFOUND',
 *   code: 'ENOTFOUND',
 *   syscall: 'queryMx',
 *   hostname: 'whatever123a.com' }
 */

checkMX('[email protected]')
  .then(console.log)
  .catch(console.error);