Pegasus on Linux.
Once upon a long ago, I was a windows user. Thunderbird had not yet been imagined. Netscape had a near monopoly on the browser market. Eudora was everyone's favourite mail client but it wasn't mine. I was a Pegasus mail user. It was and still is a lovely piece of software. Unfortunately it isn't available for linux and the author has no plans of creating a linux version. The good news is that he doesn't have to, the windows version runs perfectly on linux using Wine.