The company that I work for has a network security policy that requires me to be on the corporate VPN to connect to the Exchange servers. This means that determining whether or not Evolution can actually fetch (and send) my mail is more complicated than just determining whether or not the computer is connected to the network, because I also need to be on the VPN.

Right now there are Network Manager plugins/extensions that enable a person to connect to a VPN using NM (right now openvpn and vpnc are supported). I think that it should be possible to create per-account policies of the nature of "this account is online only if NM is connected to VPN foo." I'm not sure how much of the work in implementing this would be Evolution specific or be a more general NM/Gnome change (I'm just a user!), but improving the VPN support in Evolution/Gnome would go quite a ways in making Evolution more enterprise friendly.