simple email adress validator; checks if the server responds and has an MX or A record

email_address_validator.rb

require 'mail'
require 'resolv'
require 'net/telnet'

class EmailAddressValidator < ActiveModel::EachValidator

  attr_reader :mail

  def validate_each(record, attribute, value)
    return if options[:allow_nil] && value.nil?
    return if options[:allow_blank] && value.blank?

    begin
      @mail = Mail::Address.new(value)

      ok   = has_correct_syntax?(value)
      ok &&= is_not_banned_domain?
      ok &&= has_top_level_domain?
      ok &&= is_valid_domain?
    rescue Exception => e
      ok = false
    end

    record.errors.add attribute, (options[:message] || :email_address) unless ok
  end

  private

  def domain
    mail.domain
  end

  def address
    mail.address
  end

  def has_correct_syntax?(value)
    domain && address == value
  end

  def is_not_banned_domain?
    options[:banned_domains] ? !options[:banned_domains].include?(domain) : true
  end

  def has_top_level_domain?
    mail.__send__(:tree).domain.dot_atom_text.elements.size > 1
  end

  def is_valid_domain?
    has_domain_record(Resolv::DNS::Resource::IN::MX) || (has_domain_record(Resolv::DNS::Resource::IN::A) && is_port_open?)
  end

  def has_domain_record(type)
    Resolv::DNS.open.getresources(domain, type).size > 0
  end

  def is_port_open?
    unless (options[:no_port_check])
      telnet = Net::Telnet::new(
        'Host' => domain,
        'Port' => '25',
        'Timeout' => 5,
        'Telnetmode' => false
      )
      telnet.close
      true
    end
  end
end