The reason why you are having a challenge finding a reseller program for Microsoft hosted solutions is due to Microsoft's licensing model. That is about to change. According to redmondmag.com Microsoft has a new program call "license mobility" that makes changes to the SPLA agreements that will reduce cost for hosting providers. So in the near future these services will be available for you to resell. This included hosted Exchange, SWL server, and Sharepoint.
There are some good changes that have come down, and the 'outsourcing' licenses on SPLA are now gone which were the expensive ones. I think MS knew they were killing themselves on SALES on that one, meaning they were sending people to linux by having such a cost.
However, I think at the moment with excahnge some of the bigger players reselling games are a better sell. apptix/mailstreet are a big player with a good product and multiple options for resellers, groupspark is one that has been at several hosting tradeshows, I don't have first hand experience with them but they are a white label option.
Many times these are cheaper, with better software bundles and addons that are much more popular than some do-it-all hosting shop.
I know we tried to do exchange back several years ago, and the costs of the licenses then for a small time operation were just too much to make a profit, even at cost we were more costly than the other market players. The new license model will help a bit, but there is still a lot of competition and IaaS type providers that may just be more profitable for you to work toward integrating their product into your model.