default for Exchange 2003 =
http://domain.com/exchange
default for Exchange 2010 =
https://domain.com/exchange
first try and connect to OWA while you're on the same LAN as the Exchange 2010 server.
https://servername/exchange
once connected, install the certificate in IE/Firefox and voilah.
should work.
But if you cannot connect to OWA from outside your LAN then you have DNS issues
You need to create a Outlook Web App DNS entry for your domain
webmail.yourdomain.com -should point to- the external IP address or hostname of your Exchange 2010 server that holds the "edge transport" role.
This server should have an external IP / traffic must be routed to it via your firewall/router.
Also, RPC over HTTP only needs to be enabled in Outlook if choose to use it.
If you enable it then your Outlook clients do not need a VPN in order to connect to Exchange 2010 when they're NOT on the LAN.
RPC over HTTP will encrypt comms between Outlook/Exchange.
No RPC over HTTP = you need a VPN when your clients travel and they want to use Outlook. (and not OWA)