For years now I have been a committed, carnaptious devotee of Eudora for e-mail.
Although I love it deeply, its approach to handling attachments has finally driven me to move on. In Eudora, attachments get separated out of e-mails into an "attachments" diretory. At that point, the connection between the e-mail and its attachments is brittle. Move e-mails around folders to organize them and the attachments stay put. This results in an attachments folder that grows without bounds and a set of e-mail folders that don't quite stand-alone anymore because of references to files in the attachments directory.
A couple of days ago I followed the advise of some fellow Propylonistas who loudly sing the praises of Thunderbird.
It is indeed a wonderful thing. A veritably oasis of calm in a sea of e-mail entropy.
Firstly, it imported all my Eudora folders (and attachments) without a hitch. Secondly - it stores e-mail attachments inline so that you can move complete mails around folders.
And most impotantly - it stores mail in plain old text files using mime encoding for the atttachments (just as God intended). (Yes, there are times when I process mail with grep and Python and Emacs and...). Proprietary binary formats? Just say no!.
Its got funky spam protection stuff too. Whats not to like. Sign me up!