Settings → ListServer Settings
In VPOP3, the ListServer is an automated 'user' which handles mailing list requests, such as subscribe & unsubscribe commands.
Anyone can send an email to the ListServer to subscribe to (public) mailing lists, or unsubscribe from mailing lists they are on, or see a list of public mailing lists etc.
To do this, people would email to the <listserver name>@<your domain> with a message (see below)
See here for commands that users can send to the ListServer.
This sets the name/email address of the ListServer in VPOP3 Basic, or the default ListServer in VPOP3 Enterprise
Usually this is set to something like listserver, in which case people would send emails to 'email@example.com' to have them processed by the list server.
The ListServer reply to is the 'Reply-To' address which will be specified in messages from the list server. This is there for those people who will reply to the automated responses messages from the list server as if it is a real person - so, you could set the 'reply-to' to be an administrator, so that their replies can be seen by a real person rather than rejected as unintelligible by the list server.
If this is checked, then if someone sends a message to your list server which it doesn't understand all, then it will tell the main administrator about the message, in case a user needs help.
If someone sends a message to the list server with syntax errors in, VPOP3 will respond with an error message to tell them of the errors.
If this box is checked, then if VPOP3 finds no valid commands in the message, it will just ignore it, rather than reporting it. This is so that it will ignore spam sent to the list server, rather than informing a, probably forged, email address about the problem
In VPOP3 Enterprise, you could have multiple list servers - eg if you run several different domains on one VPOP3 server. This way, if you send an email to one list server asking which lists are available, it will only tell you about the lists which it manages itself.
You configure the extra list servers in the List -> Subscriptions configuration itself. This section here just lets you view the list servers which are configured.