If you are wanting to use any of VPOP3's advanced features (such as mailing lists, message monitoring, wildcard aliases etc) because you are LAN forwarding to a less capable server, then you will usually need to have VPOP3 do all its normal message routing functions initially, then, as the final stage have it forward the messages on to the other server.

To do this, you need to create a user account in VPOP3 for each user on the onward mail server. This means that you have to have a VPOP3 licence sufficient for all your users, rather than needing a maximum of a 25 user licence. You also need to create Mappings in VPOP3 instead of creating the aliases on the onward mail server.

In each user, in the 'Forward To' section enter something like SMTP:user@mydomain.com@192.168.1.1, tick the Use Forwarding box, and tick the Don't use forwardings or assistants if mail would be quarantined box.

This tells VPOP3 to forward messages for this user on to user@mydomain.com on the mail server 192.168.1.1 using LAN forwarding. However, if the message is spam, it won't forward it, but will put it into the VPOP3 quarantine for that user instead.

Finally, remove any LAN Forwarding rules from Settings → Local Mail → LAN Forwarding → Configuration or they will override the users and associated mappings.