Exchange 2010 Design Considerations

Hello all I have a design scenario and would like to get some opinion on how i should deploy this. Current infrastructure:

6 organizations with all their own exchange servers ranging from 2003-2010 (mostly 03). Each organization has at minimum 2 sites. All organizations will have Office 2010 when we are ready to switch them over. We will have redundant internet connections at all sites. Figure about 75 users on average per site. Figure 10mbps per internet connection.

New Design requirements/goals (if possible), FYI there will be one network with everyone connected via VPN/p2p/mpls and separate subnets.

1. Mailbox servers at multiple data centers with DAG's setup for replication
2. Ideally CAS servers at the datacenters and not at each organization, need best practices on this
3. Setup UPN's and mailboxes in the datacenter domain (maybe trusts?).

Would appreciate any input on this. Do we actually need anything on premises for any of the organizations? I am hoping to keep everything in the datacenter and again setup UPN's for those users and deploy scripts or something that would setup outlook profiles at each organization. Maybe setting up trusts would be easier? The organizations don't want to trust each other but can trust the data center's domain.

I have heard of an Exchange 2010 hosting feature in sp1. Would this add any advantages?

Thank you!

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Neil RussellTechnical Development LeadCommented:
An important consideration when talking about exchange hosting mode is not so much what you get BUT what you Don't get.  Have a read of this article and see if you had planned on using ANY of the things you dont get before you consider further.

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Something else that you should be aware of:

Organizations that deploy Exchange 2010 SP1 with the “/hosting” switch in either a production or lab environment will not receive any support from Microsoft unless they are a hosted service provider and have acquired the necessary licenses (more specifically Service Provider Licensing Agreement (SPLA) and High Volume Sales (HVS))

Simply put, you can't use (or more accurately, shouldn't use) hosting mode if you have a standard license agreement.
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