Seems as if the closing ) is missing in the query. I just chekd it in svn and it is there, maybe it has been fixed in the meantime. Does line 63 in the file /usr/local/ispconfig/interface/web/mail/mail_domain_del.php looks like this on your server?
$records = $app->db->queryAllRecords("SELECT forwarding_id as id FROM mail_forwarding WHERE source like '%@".$app->db->quote($domain)."' OR (destination like '%@".$app->db->quote($domain)."' AND type != 'forward')");