On my Drupal 6.14 your modul works fine, except for one address: info@fahrschule-wunderlich.de
I don't know why, but your moduls says it is invalid. But it isn't!

Comments

Hi,

It is because checkdnsrr function ( http://es.php.net/checkdnsrr ) returns false on domain name verification, may be no MX records or similar. Dash in domain name is not the reason.

The module has this code:

if (!checkdnsrr($host, 'ANY')) {
  return t('Email host %host invalid, please retry.', array('%host' => "$host"));
}

From the checkdnsrr manual page linked above:

Returns TRUE if any records are found; returns FALSE if no records were found or if an error occurred.

so if DNS returns an error for some reason e.g. DNS server playing up then we get a false negative...

I'm getting sensible responses for that domain name now, so I imagine it was a transient problem.