UPDATE: be sure to see this important development:
https://mikecrowley.wordpress.com/2011/08/30/hosting-exchange-2010-without-the-hosting-switch/
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Check it out:
“This download contains topics that will help you plan, deploy, and manage Exchange Server 2010 SP1 (beta) in a multi-tenant organization.”
“Microsoft Exchange Server 2010 SP1 will form part of the suite of multi-tenant capable products that will replace the Hosted Messaging and Collaboration 4.5 solution.”
There have been several TechNet threads on Address List Segregation, which this approach encompasses – but only indirectly. Deploying Exchange in the abovementioned multi-tenant configuration requires a special configuration of Active Directory. This is not something you simply choose to implement one day; but instead you build your forest to support in the first place. Exchange 2010 (with the /hosting switch) must be deployed in a new forest at Windows Server 2008 functional level.
The infamous “Address List Segregation” whitepaper is still not released. Don’t consider deploying Exchange in a hosted configuration just for this feature. I would hang on. Watch Dave’s Blog for updates.
Some other interesting points about multi-tenant support:
- You must install Exchange 2010 (SP1) from the command line in order to use the /hosting switch.
- Correction: You must disable the Exchange Control Panel (ECP) [This is optional, not required]
- The following cmdlets listed in this section are those that are only available for Hosting Deployments.
- Remove-LinkedUser
- Get-Organization
- New-Organization
- Start-OrganizationUpgrade
- Complete-OrganizationUpgrade
- Get-RecipientEnforcementProvisioningPolicy
- New-RecipientEnforcementProvisioningPolicy
- Remove-RecipientEnforcementProvisioningPolicy
- Set-RecipientEnforcementProvisioningPolicy
- Update-ServicePlan
Exchange 2010 SP1 doesn’t support the following features in Hosting mode:
- Exchange Management Console
- Public Folders
- Unified Messaging Server role
- Federation
- Business-to-Business features such as cross-premises message tracking and calendar sharing
- IRM
- Outlook 2003 support (EnableLegacyOutlook)
- Edge Transport Server role
Mike Crowley's Whiteboard 
Hello Mike,
Why Exchange 2010 SP1 doesn’t support the following feature: Unified Messaging Server role?
It does – just not in a hosted environment. Check this link: http://technet.microsoft.com/en-us/library/bb125141.aspx
Hi Mike,
After installing Exchange 2010 SP1 without the multi-tenant mode. Can I run a switch to convert to a hosted mode without a complete reinstall?
Thanks
No, sorry. You need an AD forest that has never had Exchange before. 😦
Thanks for the response. Another question I had was that is it true that the multi-tenant mode won’t provide the management console?
See here:
Since Exchange 2010 SP1 in hosting mode doesn’t allow the usage of the Exchange Management Console (EMC), it’s and recommended hosted service providers develop their own control panel and provision the relevant UI solution on top of it. With that said though, you can of course access the Exchange Control Panel (ECP) as the Administrator of a specific multi-tenant organization, and then manage the organization via the options available in ECP. For instance, when running Exchange 2010 SP1 in hosting mode, you can do basic management tasks such as create mailbox users, distribution groups, and contacts. You can also edit existing user mailboxes and reset passwords etc.
http://social.technet.microsoft.com/wiki/contents/articles/exchange-2010-sp1-information-for-hosted-service-providers.aspx
Question on Exchange 2010:
Can Address list segragation be done without installing 2010 in hosting mode?
When SP2 comes out. Yes. Read here: http://blogs.technet.com/b/exchange/archive/2011/01/27/3411882.aspx
What the advantages of a Multi-Tenant mode? Sound like a more Disadvantages of this mode.
Ricardo, the “advantages” are that you can host multiple tenants!! But your right, there are limits. This is why I am recommending to view the article at the top and consider ABP instead.