Seeing that the Exchange 12 CTP build has been available for some time now, we have learned that some of our customers that are not on Exchange 12 TAP program have been installing Exchange 12 into their production environment.
Unfortunately, those experiments came to us as problems that affected production Exchange servers. While we have been working to fix those situations - it is an unnecessary problem that could have been avoided.
If you have access to Exchange 12 CTP build or even are a part of the Exchange 12 Beta program, DO NOT install Exchange 12 into your production environment. Exchange 12 will extend your Active Directory schema and it will change permissioning on Exchange organization and other objects which, in this early stage, might cause interoperability issues.
Just to give you a little bit more information: with our TAP customers, we go through a very specific process to make sure that there are no issues when Exchange 12 is joined into production. We review a lot of information about each and every environment, and even then, some customers can not install our current Exchange 12 builds into their production environments yet. Exchange 12 is Beta code. As such, it has not been extensively tested in all possible configurations and results of installing it into the existing Organization can lead to unexpected results.
We want to strike a balance between releasing early builds so that our customers can see the product and give us feedback and ensuring that our customers have a good experience with the product. When our instructions about an early build being for testing only are ignored, customers can have a bad experience and it makes it harder for us to decide to release early builds because we don't want those situations to happen.
What you of course can do, is install Exchange 12 in the lab and test it there... you can test coexistence too (meaning, having Exchange 2003 servers in the same Org) but this should be in your labs only.