d3ad wroteyou need to check whether you exchange servers allows RPC over HTTP, that is the only way to connect to your exchange sever over internet (without having a VPN)
If you are the administrator of the exchange sever there are alot of considerations on allowing RPC over HTTP it comes with a bandwagon of vulnerabilities.
I would highly recommend going towards the VPN Route, if thats not possible for you i recommend deploying OWA, via DMZ ISA server model.
I can't access the server, can't configure it nor reach it through VPN
My only question is : Why is it working on ActiveSync (without VPN or anything) and not working on pc ?
is there an activesync software on pc ?