Microsoft was an early and intense adopter of email. -An intentional understatement
Speaking as the dev lead for the Exchange Directory (1991-1996) and later on Active Directory (1996-2005), there's a lot wrong with this chapter. NT's approach to functional directory services in the early 90's was "wait for Cairo. they're building one", which meant that we in Exchange had to build our own directory service. When Cairo collapsed (late 1995) Exchange and NT struck a deal so that once Exchange 4.0 shipped (April 1996) one of my developers and I brought a copy of the Exchange Directory source code over to Windows, and we built Active Directory out of that. Exchange in no way "bet on" the NT Directory; we essentially built the replacement for it in order to get the features we needed. Ask me if you need details.
However, the part about endless repeated pressure to build everything (specifically including the directory) on top of SQL is entirely accurate.
I'm only moderately annoyed that I had to pay ten bucks to post this correction.
I worked at Novell during this period (1995-1999), in the GroupWise team. Exchange was our competition. GroupWise and Novell Directory Services was better, but tied to a sinking ship.: Netware.
"The virtual cycle between these products" possibly edit to "virtuous circle"