Port 587 (SMTP submission) — check if it's open

Port 587 — SMTP submission. Mail submission from clients (STARTTLS).

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 587). 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.