Add a custom MX validator to your rails app in app/validators/email_mx_validator.rb

email_mx_validator.rb

# frozen_string_literal: true

require 'mail' unless defined? Mail
require 'resolv' unless defined? Resolv

# == Summary
# This validator checks for a valid DNS MX resource
# for the given email address. The domain of the email
# address is extracted and used to perform a DNS lookup
# for an MX resource.  A valid DNS MX resource does not
# gaurantee that the email address is valid, only that
# the *domain* of the email address is valid for email.
#
# == Usage
# Use the +EmailMxValidator+ on a +User+ model.
#
#     class User < ApplicationRecord
#       validates :email, email_mx: true
#     end
#
#     user = User.new email: 'scott@serok.us'
#     user.valid?
#     #=> true
#     user.email = 'scott@wizards.unicorn89'
#     user.valid?
#     #=> false
#     user.errors.full_messages
#     #=> ["Email MX record missing for the domain \"wizards.unicorn89\""]
class EmailMxValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    domain = email_domain(record, attribute, value)
    return false if record.errors[attribute].any?
    mx_record_for_domain(record, attribute, domain)
    return false if record.errors[attribute].any?
    true
  end

  private
  
  def email_domain(record, attribute, email)
    address = Mail::Address.new(email)
    return address.domain if address.domain
    record.errors[attribute] << %("#{email}" does not have a domain)
  end

  def mx_record_for_domain(record, attribute, domain)
    dns = Resolv::DNS.new
    dns.getresource(domain, Resolv::DNS::Resource::IN::MX)
  rescue Resolv::ResolvError
    record.errors[attribute] << %(MX record missing for the domain "#{domain}")
  end
end