They do fully integrated enterprise commications: * Mail (often with proprietary additions, also shared mailboxes) * Calendar (appointments, mostly: plan with

They do fully integrated enterprise commications: * Mail (often with proprietary additions, also shared mailboxes) * Calendar (appointments, mostly: plan with others’ free/busy times, invitations, invitation replies, integration with resource booking, …) * Global Address Book * Tasks (To-do list, can also be mailed) I don’t know any non-proprietary solutions that integrate all this in a decently user-friendly way. Google certainly does, but then you might as well stay with Microsoft. It’s not just about the service capabilities either: The client (be it a web or a desktop application) has to be good as well.