Qualcomm just released Eudora 6.0. The version 6 adds a spam filter that has been working incredibly well for me during all the beta period, a new feature called Contents Concentrator that's pretty smart too, a new look, mailboxes drawers, and more.
I started using using Eudora ages ago, it seems (I was running system 7, I think, so that's not yesterday) and although I've tried about every other emailer that came out since then, none of them led me to switch. To me Eudora has always been the most compatible, the fastest and the best at searching, sorting and managing years of emails. And it has no database to corrupt! ;)