We really should migrate away from filter_var to a stronger solution like http://code.google.com/p/isemail/source/browse/PHP/trunk/is_email.php which seems like the only solution mentioning RFCs, being maintained, and not being dependent on a specific PHP version.

I emailed the author to see if he would be willing to migrate it to a more class-based approach and remove all those constants from the global namespace.