Giles Turnbull asks what I want in a new email client for the Mac. http://www.oreillynet.com/mac/blog/2006/06/what_would_you_like_from_a_new.html
I use Eudora for a lot of email management. Eudora’s limits needs to be fixed: (1) More than 32,000 messages in a mailbox. (2) Personalities should be able to be renamed safely. (3) better handling of filter rules that include the moving of attachments to another folder. If you move the Eudora Folder location, all the filter rules break, and your attachments all end up in the Attachments folder. Boo! And the mail client must save mail in mbox format. I occasionally use various command line tools to get stats on my mail.