for some reason webmail service is not available for the public for outside the office... but if you are on the same network you are able to get in...but is online for all users on the network
I have checked the event viewer and notice it has this event ID 116
Microsoft Exchange OLEDB was unable to initialize event system correctly. Support for store event may have been disabled. HRESULT = 0x8000ffff.
Usually Webmail works or doesn't work and its ability to work outside the network is dependent upon DNS and firewall settings.