A lot of "fun" with those documents is that some of them quite probably were archeological digs on Microsoft side too.

For example, the MAPI specs have references to valid parts of the protocols and data structures that are not used anywhere and which in fact crash MAPI libraries (so Outlook and Exchange just throw errors if you give them such data), sometimes giving a glimpse of how there might have been abandoned features that were never delivered.

Like, surprisingly, HTML email support in Outlook[1] :D

[1] MAPI Message struct, and thus Exchange and Outlook, crashes when encountering "HTML message" let's call it "submessage". Turns out the valid way to save a HTML message is by wrapping it in RTF and saving it as Rich Text submessage. Plain text is another submessage.