You need to set up a DNAT rule: Any|Port 443| Public Exchange Address forwarded to your exchange server internally, with auto rules.
Note that Exchange actually uses HTTPS (443) not HTTP (80).
Of course you will have to have set up:
1 the server to handle RPC over HTTP (Microsoft KBs explain this)
2 certificates on the server
3 clients to use HTTPS
Alternatively, or in addition, you might want to consider setting up SSL VPN tunnels so external users are authenticated to the Astaro prior to communicating with your exchange server.
