Insert row in external database

I want to insert data into the table ‘cls_users’ of an external Mysql database.
I started with this:

try {
		$cmsadb = new wpdb(CMSA_USERNAME, CMSA_PASSWORD, CMSA_DATABASE, CMSA_HOSTNAME);
		$cmsadb->show_errors();
	} catch (Exception $e)	{
		writeLogWP("Connection error: " . $e->getMessage());
	}

with no error; then I tryed to insert into table ‘cls_users’:

$name = 'john';
$email = '[email protected]';
$result = $cmsadb->insert('cls_users',
			array(
				'name' => $name,'mail' => $email));

But got $result = false.
What am I do wrong? Thanks