Eudora had its own very distinct take on mail client UI. Many loved it. I never really got on with it, although I could use it.

While the native codebase is probably too old to salvage now, there was a project to write a Eudora-style UI for Thunderbird as an add-on. That might be easier to revive for 21st century email.

http://www.staroceans.org/wiki/A/Eudora_OSE

https://www.majorgeeks.com/files/details/eudora_ose.html

I know people that used it because it was self contained for Windows if I remember correctly. I remember one person running the installation off of a Zip drive back in the 90's. I warned them that Zip disks like to randomly self destruct and he'd better be making backups.