If you're getting the “Unable to send e-mail. Please contact the site administrator.” error on a Drupal site that's hosted on an Ubuntu server, there's a good chance you don't have postfix installed. Resolving this is quite simple:

sudo apt-get update sudo apt-get install php-pear sudo pear install mail sudo pear install Net_SMTP sudo pear install Auth_SASL sudo pear install mail_mime sudo apt-get install postfix