gistfile1.rb

require 'resolv'

class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    if Resolv::DNS.new.getresources(value.split("@").last, Resolv::DNS::Resource::IN::MX).empty?
      record.errors[attribute] << (options[:message] ||  "does not have a valid domain")
    end
  rescue Resolv::ResolvError, Resolv::ResolvTimeout
    record.errors[attribute] << (options[:message] ||  "does not have a valid domain")
  end
end