Exchange Server 2010 SP1 contains new functionality for Hosting Providers. Using the /Hosting switch during the unattended setup it is now possible to create a multi-tenant Exchange Server 2010 environment.
Please note that this is not a replacement, nor a good alternative, for the Address List Segregation. If you need an Address List Segregation solution, please keep an eye on Dave Goldman's weblog: http://blogs.msdn.com/b/dgoldman/archive/2010/09/09/the-future-of-exchange-2010-address-list-segregation.aspx
You can setup a multitenant Exchange Server 2010 SP1 environment using the unattended setup:
setup.com /mode:install /roles:ca,ht,mb /Hosting
And you have to use the /Hosting switch on any subsequent Exchange Server you'll install.
After installation you first have to create a Mailbox Plan and an Offering. These are Hosting specific things and are used when creating different companies, also referred to as tenants or sometimes organizations, in your Exchange environment. You can find these in C:\Program Files\Microsoft\Exchange Server\V14\Client Access\ServicePlans. If you edit the sample plans in this directory, there's always a chance the files will be overwritten during a servicepack in the future.
After creating the Mailbox Plan and Offering you can create a new company using the New-Organization cmdlet. In this cmdlet you'll create a new tenant, assign an Offering and Mailbox Plan, an SMTP Domain and create an Administrator with a Mailbox in this Organization. For every company you host you have to create a new Organization.
Now this Administrator can logon on Outlook Web App and the Exchange Control Panel and yes, there's a "New Mailbox" functionality in the ECP. The Administrator can now create new Mailboxes in his own 'private' Organization. And these Organizations are fully seperated from each other, so each Organization has its own Global Address List for example.
But, this Exchange implementation is very, very limited. For example, there's no support for Public Folders, no Outlook 2003, No Unified Messaging, No Federation etc. Oh and by the way, there's only an Exchange Management Shell, there's NO Exchange Management Console. FOr fully utilizing the multitenant solution in Exchange Server 2010 you have to use a Dashboard in Exchange to offer all kinds of services to the various tenants.