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.
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.
Used to rely heavily on this, until an upgrade on one of my systems blew up the entire profile and I just stuck with webmail mostly. On my phone I have gmail, Outlook and Thunderbird (forked from K-9).
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