I discovered a simple quirk of setting up eMule (P2P) on Windows 10 that causes port forwarding to fail with AirVPN. This tip that may save others a lot of time. During setup eMule creates an inbound rule in Windows firewall that applies to private networks in scope. This makes sense if using eMule without a VPN. However, Windows treats the AirVPN as a public network. Consequently port forwarding with eMule and AirVPN will not work using the default inbound rule created during eMule setup.