I miss Eudora. Then along came Apple's Mail.app which was "good enough" and too cheap and easy so it killed the competition.
Maybe Apple is deliberately screwing up Mail.app to invite return of 3rd party mail clients?
Deleting Classic Layout and sortable re-arrangeable re-sizable columns is only the latest gaff.
Prior to that Apple decided contrary to decades of practice that the attribute line on replies should be quoted. And they started putting .signature above the quoted text. Both are dead wrong.
Before that Mail.app didn't have threading so Apple invented "conversations" based on the Subject: header, never mind the wonderful existing solution (that even Outlook gets right) of using Message-ID, References, and In-Reply-To headers. At least Apple sets In-Reply-To, haven't looked lately to see what they do with References but if there is a References in the message being replied to they should be able to add another Message-ID to the list no matter they otherwise ignore the header.
I miss Eudora, which got all of the above right. But if I recall correctly the end of Eudora was about the same time as the rise of IMAP4. And before iCloud. Eudora didn't sync message rules between one's multiple machines. Didn't work off a common Contacts/Address book.
Maybe I should return to mutt, vim, and procmail in a Terminal window?