Cory's horrible tale is fairly common. In fact, it is more common on Windows machines than it is on OSX because Windows programs use the very-shaky Windows Registry which is easily corrupted and/or overwritten.

I use Eudora for my daily mail, and have been using it forever. It is adequate-to-good, and I don't dislike the UI as much as others do. I'm seriously thinking of switching to Mulberry because many of the Mac-using Internet experts I know use it. If you are an IMAP user, Mulberry is considered one of the best, and it's supposed to be just fine with POP as well. I'll do some tesing this month to see if I want to migrate. For those of you who don't mind text-oriented UIs, you can't go wrong with Pine. It is still well-supported (thank you University of Washington!), is very predictable, and handles all the latest protocols.

And a hint for everyone: if you have direct control over your mail server, or are friendly with your server administrator, you should be keeping a copy of every piece of email that comes in for you. Simply have the incoming SMTP server save one copy of every message to your archive and another to your "real" mailbox. Disk space is essentially free these days. I get a buttload of mail and more spam than most people, and my archive-of-everything for the past year is about .6 gigabytes, or about $1 worth of hard drive.