Eudora was open-sourced in 2018.

see https://computerhistory.org/blog/the-eudora-email-client-sou...

and from https://en.wikipedia.org/wiki/Eudora_(email_client)

  The last 'mainline' (pre-OSE) versions of Eudora for Mac and Windows were open-sourced and preserved as an artefact by the Computer History Museum[2] in 2018; as part of the preservation, the CHM assumed ownership of the Eudora trademark.

  The only actively maintained fork of the software, known as Eudoramail as of June 2024, originates from 'mainline' Eudora for Windows as preserved by the CHM. Hermes, its current maintainers, describe Eudoramail 8.0 as currently being in alpha; Wellington publisher Jack Yan, meanwhile, points out its stability, a number of well-characterised and reproducible display bugs notwithstanding.

  On May 22, 2018, after five years of discussion with Qualcomm, the Computer History Museum acquired full ownership of the source code, the Eudora trademarks, copyrights, and domain names. The transfer agreement from Qualcomm also allowed the Computer History Museum to publish the source code under the BSD open source license. The Eudora source code distributed by the Computer History Museum is the same except for the addition of the new license, code sanitization of profanity within its comments, and the removal of third-party software whose distribution rights had long expired.

The time period under discussion ("before Thunderbird", and the heyday of Outlook lock-in, and I would also add before gmail) is well before 2018.

I used mutt at the time too, but I don't think it's in the same category as the graphical clients. For a while Gnome's evolution was also big in free OS circles.

Eventually, and I was glad to see it!, but way too late for it to matter much. I would've used Eudora when it was originally offered. Since I couldn't, I got comfortable with Thunderbird. And when my friends who used Eudora had to migrate off of it, I set them up with Thunderbird, too.