I 've been using Eudora since 1997, and running 6.3 on my iMac with Snow Leopard. Without many problems, carrying around f.i. an inbox with more than 8000 messages. Eudora not running under MacOS Lion/Mountain Lion, is one, if not *the* major reason, I don't upgrade my system. Using Eudora for collecting and keeping messages within fast and easy reach, is something I cannot do without - both privately and surely for my job.
What I would mostly miss when migrating to another email-program, is twofold:
1) being able to sort mails by date *and* having them grouped at the same time by subject (thread);
2) the navigation through #mails: pressing spacebar will scroll through an open window, closing it and opening the next message in the same thread when having reached the end of that mail. That is just superior navigation to me, and I cannot imagine myself having to click and scroll through my mails another way.
I am not aware of any other program offering these; I 'm looking at Mailforge as well (because of their claim to more or less replicate Ye Olde Eudora, but it lacks the navigation...