Possibly try:
if ( mysql_query( "$sql", $link ) )
Also, as you haven't included your table structure for "email", I will have to assume that you are entering exactly the correct number of column entries in your sql.
Again, another possibility: Where's your db password as there is no sign of it in your above script. Even if connecting as root, a password IS required.
$db = mysql_connect($dbhost , $dbusername , $dbpw);
mysql_select_db("$dbname",$db);
Slightly off-topic, but you might want to consider adding a simple check to prevent duplicate addresses being added to your db aswell.