@nprofile1q... I know the answer, or at least part of it. I wrote the first one of these — NetNewsWire Lite 1.0 was released in 2002, and it was the first RSS reader to resemble an email app.
I was actually thinking about Usenet, not email, but whatever — the question I asked myself then was how would I design a Usenet app for (then-new) Mac OS X in the year 2002? The answer was pretty clear to me: instead of multiple windows, a single window with a sidebar, list of posts, and detail view.
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