index.js

var mailgun = require('mailgun-js')({apiKey: api_key, domain: domain});

exports.sendWelcomeEmail = functions.firestore.document('xxx/{userId}').onCreate(event => {
  var user = event.data.data();
   var name = user.Name;
   var email = user.Email;
     var data = {
    from: ' App xxx',
    subject: 'Welcome to Tools Android App!',
    html: `HTML template`,
    'h:Reply-To': 'noreply@xxx',
    to: email
  }
  if( email ) {
     mailgun.messages().send(data, function (error, body) {
    console.log(body)
  })
}
    });