Ever since I first got onto the Internet in 1994, I have been using Eudora as my email client. (That's because it was part of the software collection the ISP provided to me, along with Netscape Navigator, WS_FTP, and I can't recall what else. I still use that FTP program, as well.)
Qualcomm maintained Eudora for many years, and then gave up on it, but opened the source code. The Mozilla foundation took it over, creating Thunderbird for Linux, Mac, and Windows. I've been trying to like Thunderbird, but still prefer the aging Eudora. Thunderbird is similar but too different to be comfortable for me. Just like Firefox is just not as comfortable to me as is the full-featured Opera Web browser. Also, Thunderbird does a poor job of importing Eudora mailboxes, and I have tens of thousands of email messages that I'd like to access.
Imagine my surprise when I learned that Qualcomm is back at it again. They are creating a Eudora version of Thunderbird -- named Penelope that's essentially a skin that lets us Eudora users feel at home. More info here: https://wiki.mozilla.org/penelope