No, Exchange does not use MSSQL. It uses an ISAM engine called the Extensible Storage Engine (code named Jet Blue). There is no SQL executing inside of Exchange. It's all raw low-level ISAM.
The same codebase is also used by Microsoft to host Active Directory, the centralized password and security database that is hosted on Windows Server domain controllers. (I happen to know this because I own an patent on using SQL for I&A.)
ESE trades off the flexibility of SQL in favor of speed and robustness. It has excellent data logging and error recovery capability, a good thing in something as mission critical as your organization's central security database.
ESE allows for continuous backup capability, where transactions can be archived in real time without shutting down the mail server or interrupting mail traffic. This is a big feature of Exchange, particularly in today's hyper-litigious where a permanently searchable mail archive for legal discovery is pretty much a necessity for every organization.
Yep...
I havent managed email servers for quite some time now. The last design I was involved in for Exchange 2010 required an HA cluster of MS SQL server. I don’t do DB admin, I just prepped them. I know they were required, that’s all