We couldn't make Outlook 2013 (latest updates) to connect to our Exchange 2013-SP1 through the Internet.
Any PC inside the LAN (on the domain or outside the domain) can connect normally.
Any PC outside the LAN (WAN) cannot connect, it keep showing this error:
"The action cannot be completed. The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action."
OWA is working fine from the WAN, smartphones (Android & Apple) are also working fine from the WAN.
Port 443 is open on the firewall.
So what we are missing!