Listserv Control
The Listserv Control feature is used to allow or disallow the last server that is sending you an email. You can control listserv and mailing lists with this functionality. Even though a users email might be userx@domain.com, the actual server that is sending the message might be computer.someotherdomain.com. If you accept that server you will get all the mail from that server, regardless of what the return email address is. That is how listservs and mailing lists work.
As with the server or domain control, this is the default setting, and an individual accept or deny overrides the default settings.
To use this feature, you can select the List Servers button on the Email Server Control Screen and enter the server in the add new server box. To help identify these servers, you can use the create list feature of the bottom window. Once you click Create List, you will need to exit the email control screen and the manage addresses, and check your mail. KnockKnock will continue creating your list for up to 10 account checks, you can come back whenever you feel you have enough information.
There is no need to have the same server in both the Email Server settings, and the List Servers settings. Either one is fine, although spammers may change their domain name, the last server may stay the same.
Spammers sometimes use special programs on their local computer, and you will see weird server names in the list view. You can almost always deny these and KnockKnock will still control these senders.