Going back in time - I have someone who is SBS 2003 (IIS 6). OWA fails with 404 error externally over HTTPS. here are some clues:
1. HTTP works Externally
2. HTTP & HTTPS works internally
3. Firewall has port 443 open to the server
4. Dont think it is FW because you get "Untrusted Cert" warning prior to 404 error when accessing HTTPS externally (could be wrong) - saving the valid cert issue for later.