Postbox is a mail client for Windows and Mac built over the Mozilla (Thunderbird) codebase, with a few new features and a great deal of polish.

I’ve been a Netscape Mail user for a long time, then a Thunderbird user (since 0.8 or so), and I’m using Postbox for a week and already love it. The best feature in Netscape Mail and its descendants is the threaded mail view, and Postbox takes it to another level.

Lots of Eudora orphans are raving about it. I just bought my license; hope they keep improving it, like updating the compatible Lightning extension to the latest version.