In app/config/mail.php set the line with "from" to the following:
'from' => array('address' => 'info@' . str_replace('www.', '', Request::getHost()), 'name' => 'Yourname'),Use Request::getHost() for the whole domain (includes www if applicable). Request::getHttpHost() also includes the port number, Request::root() includes the protocol as well.
You can remove the str_replace() if your domain doesn't contain www.