Ernie Smith has a great history of Eudora, one of the most important computer applications of the 1990s.
In early 1997, two applications were in the process of taking over the internet, and both had roots in the University of Illinois at Urbana-Champaign. One of those applications, Netscape, became a bedrock of how we surf the web. The other, Eudora, put a graphical twist on email.
I wasn't much of a Eudora user myself. I envied the powerful filtering abilities it possessed, but I never much cared for the UI.
My client of choice in that era was Claris Emailer, which was both gorgeous and functional. These days I use MailMate and Mail.app (the latter because MailMate doesn't support Exchange.) If you look at those apps (and really, most other email apps, including gross Outlook) the influence of Eudora is undeniable.