Ok, then the domain names are not connected. Have you disblaed username prefixes? If employee is a linux username, then it will catch emails from all domains where the email address starts with employee@.... To prevent that, edit the file /home/admispconfig/ispconfig/lib/config.inc.php and cahnge the line:
$go_info["server"]["postfix_config"] = 1;
$go_info["server"]["postfix_config"] = 2;
then edit the mailbox settings of a user in ispconfig, e.g. change quota and click on save so that the configuration file gets rewritten.