Historical Review
Flat files without any real index containing just user ids (old, OLD, MHS)
Flat files with poor performance (DG’s CEO)
Flat files w/unbelievable performance (cc:Mail)
Non-flat files w/unpredictable performance (Notes)
Not at all, flat files with sadly predictable performance (ALL-IN-1)
Files no one one understood the layout or performance thereof (PROFS)
…of course, X.500, and, purists take a deep breath, LDAP