Ninja forms validate email

main.js

// On Document Ready...
jQuery(document).ready(function ($) {
  if (typeof Marionette !== 'undefined') {
    var myCustomFieldController = Marionette.Object.extend({
      initialize: function () {
        var submitChannel = Backbone.Radio.channel('submit');
        this.listenTo(submitChannel, 'validate:field', this.validateRequired);

        var fieldsChannel = Backbone.Radio.channel('fields');
        this.listenTo(fieldsChannel, 'change:modelValue', this.validateRequired);
      },
      validateRequired: function (model) {
        if (model.get('type') === 'email') {
          let value = model.get('value');
          if (value.indexOf("@gmail.com") !== -1) {
            Backbone.Radio.channel('fields').request('add:error', model.get('id'), 'email-domain-validate-field-error', 'Sorry, but at the moment we do not support the use of email addresses from the "gmail.com" domain. Please choose another mail service.');
          } else {
            Backbone.Radio.channel('fields').request('remove:error', model.get('id'), 'email-domain-validate-field-error');
          }
        } else {
          return;
        }
      }
    });

    new myCustomFieldController();
  }
});