in remote API examples the mail_spamfilter_user_add.php has a little typo (policy_ID)
$params = array( 'server_id' => 1, 'priority' => 3, 'policy_ID' => 1, 'email' => 'hmmnoe@test.int', 'fullname' => 'Max Mustermann', 'local' => '' );
doesn't work right because policy_ID should be policy_id