# We assume that @mx contains something, since the die()
# during MX resolution would have killed us by now
for (@mx)
{
   # It *would* be more elegant to test the primary server
   # in the event of round-robin MX or pick one in the 
   # event of load-balanced MX, but I want to test for
   # any disparities in replies.
   my @mailservers = $rr->exchange;
   for my $mailserver (@mailservers)
   {
      # Connection plus EHLO
      my $smtp = Net::SMTP->new(
        Host => $mailserver,
        Hello => some.server.tld,
        Timeout => 30,
        Debug => 0
      ) or die "Cannot connect to $mailserver!\n";
      # MAIL FROM:
      $smtp->mail('me@somewhere.com');
      # RCPT TO:
      my $response = $smtp->recipient($address);

      # Now go looking for response codes. On my system,
      # 1 = user valid, 0 = user invalid
      if ($response == 1)
      {
          # Do something, or return() something
      }
      else
      {
          # Do something, or return() something
      }

      # Be polite, RSET and QUIT properly
      $smtp->reset();
      $smtp->quit();

      # Slow down the rate between probes
      sleep(2);
   }
}