I wish Eudora was still around. The source code is available, my secret dream is to work on somtimes in the summer...
https://computerhistory.org/blog/the-eudora-email-client-sou...
https://computerhistory.org/blog/the-eudora-email-client-sou...
There's hope that the HERMES Mail project, which was formed after the Eudora source code was release, will one day be a viable replacement:
I used that until I moved to Linux, and I used sylpheed on top of maildir.
I've used Thunderbird now for years, but I also use fastmail as the mail server and use offlineimap for backing fastmail up locally.
I've got email archives going back to 1998 (which is probably when I moved off Windows NT, come to think of it).
Still, if you ever do dive in I'm up for doing some beta testing!