make dns validation optional

Evidently the checkdnsrr() function was not part of PHP until 5.3.0 was released. You'll either need to upgrade your PHP, or install the PEAR class » Net_DNS. Further details here:
http://php.net/manual/en/function.checkdnsrr.php

Marking as a feature replace to make dns validation optional.

PHP 5.3 isn't compatible with Drupal 6 either, so not an option.

i think DNS validation should be optional for another reason:

We use postini.com for a number of clients as a great effective spam filter. However our clients domain MX record is pointing to Postinis servers, which causes the DNS validation to fail. So at present we can't use the email inbox checker facility to map emails from a certain domain to a certain client.

Drupal 6 is compatible with PHP 5.3 as of Drupal 6.14:
http://drupal.org/drupal-6.14

And you can install the Net_DNS Pear class with earlier versions of PHP.

Regardless, I'm not opposed to making this a configurable option, I've simply not had time to implement it yet.

Updating title, as this clearly isn't just a Windows issue.

Fix committed:
http://drupal.org/cvs?commit=330202

Neglected to close issue after fix was committed.