I wanted to highlight this section from the new HMC 4.5 documentation:
There are two types of service providers who can make money from selling Microsoft Hosted Messaging and Collaboration version 4.5 based services, those who run the service themselves and those who are resellers.
Research shows that achieving a customer base of several thousands MAPI seats with monthly growth rates of 100's of seats is the minimum level to achieve profitability in running the hosted Exchange platform as a commercial service provider and partners that will not achieve this level of customer adoption would be significantly more profitable through a reseller arrangement with a large scale white-label provider.
For service providers that run the services themselves, to run the service profitably, the key points are:
For service providers who are not going to have the scale identified above, Microsoft strongly encourages service providers that cannot achieve at least a 10,000 mailboxes customer base to seriously consider shifting away from the operational aspect of the hosted Exchange business and explore working with one of our white-label exchange service providers that can be found at Hosted Exchange Server Partner Directory.
This means that they will not run Microsoft Hosted Messaging and Collaboration version 4.5. Instead, they will find a provider with whom they can partner and sell the service run by that partner.