I drupal'ized the validate_email function found at http://yser.de/validate_email_php.tar.gz . The attached "module" could be used or check_email() can be integrated into user.module. Unfortunately the PHP getmxrr() is not implemented on Windows platform. My user_validate_mail() in user.module looks like this: function user_validate_mail($mail) { global $base_url; if (!$mail) return t('You must enter an e-mail address.'); if (!valid_email_address($mail)) { return t('The e-mail address %mail is not valid.', array('%mail' => "<em>$mail</em>")); } else if (function_exists('check_email')) { $host = parse_url($base_url); $host = $host['host']; if ($check = check_email($mail,$host)) { if ($check[1] != 1) { return t('The check of e-mail address %mail returns:<br />%res', array('%mail' => "<em>$mail</em>",'%res' => "$check[0]")); } } } } Cheers, Karsten