> the world of desktop mail clients

We live in that world still.

> but basically during the height of MSFT dominance there was only one real mail client: Outlook.

On Windows, you had:

* Netscape Suite (later Seamonkey)

* Eudora

* Pegasus

and (edit:) two of those still exist. Plus, Outlook cost money (unless you used Outlook Express), while Netscape was gratis, and on Linux and most Unix variants, Outlook has never even existed. On Linux specifically there's Evolution and there's KMail.

And I'm sure I'm forgeting a few others.

> Then Thunderbird arrived on the scene

It was a development of the MailNews component of Netscape, to use the same XUL-based platform as Firefox. So, an evolution, not a revolution.

(edit:) Oh, look what I found!

https://missive.github.io/email-apps-timeline/

uncheck 'Web', iOS and Android.

I loved Pegasus. Specifically because to move it to another machine you just had to copy the PMAIL folder and make a shortcut. No registry awareness, no dependencies.