POP3 over TLS.
Defaults to your IP: 216.73.216.200. Up to 12 ports per check; ranges up to 10 ports (e.g. 8000-8010).
Port 995 — POP3S. POP3 over TLS.
1) Make sure the app listens on 0.0.0.0, not 127.0.0.1. 2) Allow the port in your firewall (Windows Defender Firewall inbound rule; Linux: ufw allow 995). 3) Forward the port on your router to your computer's LAN IP. 4) Make sure you have a public (non-CGNAT) IP. 5) Verify it here.
Only open it if the service must be reachable from the internet, keep the software patched and use strong authentication. Put databases, SMB, RDP and admin panels behind a VPN.