Maybe everyone already knows this, but I discovered yesterday (by mistake) that if you option-click on an email in a mailbox, Eudora will find, highlight and group together, all emails from the same sender in that mailbox. This makes it much easier to move, delete, and manage your email.
I've been using this app for eight years and keep stumbling upon nice little surprises like this one.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040910160841670