Validate email addresses.

user.rb

class User

  # pathetic
  validates_length_of :email, minimum: 6 # "x@x.cc"

  # minimum
  validates_format_of :email, with: /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,6}$/i, message: "is not a valid email adress."

  # legitimate
  validate :email do |user|
    # MTA's will attempt delivery to MX, then AAAA, then A records if present.
    local_part, domain = user.email.split('@')
    if Resolver(domain, Net::DNS::ANY).answer.select { |rr| [ 'MX', 'AAAA', 'A' ].member? rr.type }.size == 0
      errors[:email] = 'email domain is not valid.'
    end
  end

  # extreme
  validate :email do |user|
    local_part, domain = user.email.split('@')
    answers = Resolver(domain, Net::DNS::ANY).answer
    
    # Grab all the MX's and sort them in reverse order.
    servers_to_try = answers.select { |rr| rr.type == 'MX' }.sort { |a,b| b.preference <=> a.preference }.map(&:exchange)

    # Next add any IPv6 hosts, if we have IPv6 connectivity.
    if local_ipv6_address
      servers_to_try += answers.select { |rr| rr.type == 'AAAA' }.map(&:address)
    end

    # Next add any IPv4 hosts.
    servers_to_try += answers.select { |rr| rr.type == 'A' }.map(&:address)

    accepted = false
    servers_to_try.each do |server|
      begin
        Net::SMTP.start(server, 25) do |smtp|
          smtp.open_message_stream('bot@kimonoapp.com', [ user.email ])
        end
      rescue Exception => e
        if (e.is_a? ArgumentError) && (e.message == 'message or block is required')
          accepted = true
          break
        end
      end
    end
    errors[:email] = 'is undeliverable.' unless accepted == true
  end

  private

  def local_ipv6_interface
    # turn off reverse DNS resolution temporarily
    orig, Socket.do_not_reverse_lookup = Socket.do_not_reverse_lookup, true
    begin
      UDPSocket.open(Socket::AF_INET6) do |s|
        #s.connect '2404:6800:4006:802::1012', 1 # ipv6.google.com
        s.connect 'ipv6.google.com', 1
        s.addr.last
      end
    rescue
      nil
    ensure
      Socket.do_not_reverse_lookup = orig
    end
  end

end

Awesome, thanks!

It's okay. I should mention that this can be a bit brittle if your network connectivity is less than stellar.