http://msexchangeguru.com/2015/02/16/exchange-basics/
- 2 or more server which you are willing to add to an Exchange cluster should be running on same OS and same Patching level of OS. Then they should be at same Exchange version and same patching level.
- n any coexistence of exchange, once you uninstall the last exchange of legacy then you can’t install legacy version again.
- Any new Exchange update should be tested and, in any case, verify each details published by Microsoft.
- You need CAS role in every AD site where you have planned to install mailbox server.
