I have a field which has the email validator assigned to it. If I type in a partial email address like 'my.name@' it produces a warning:

Warning: checkdnsrr() [function.checkdnsrr]: Host and type cannot be empty in /var/www/myproject/lib/Doctrine/Validator/Email.php on line 114

I've tested this on Doctrine versions: 1.0.7, 1.0.8 and r5567.

Some additional info:

php -v
PHP 5.2.9 (cli) (built: Mar  3 2009 22:09:12) (DEBUG)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2009 Zend Technologies

apachectl -v
Server version: Apache/2.2.11 (Unix)

mysql -V
mysql  Ver 14.12 Distrib 5.0.67, for debian-linux-gnu (i486) using readline 5.2