I don't know what OS you're running, but on Linux the biggest contenders seem to be KMail (for KDE) and Claws-mail (formerly Sylpheed). I've used the latter bef

I don't know what OS you're running, but on Linux the biggest contenders seem to be KMail (for KDE) and Claws-mail (formerly Sylpheed). I've used the latter before, and it seems to primarily be a lighter-weight Thunderbird clone. I haven't used KMail. mutt and pine (or alpine) running in a terminal are fairly cromulent alternatives to those who don't mind navigating with the keyboard and working modally, but that's not really how many of us prefer to work with our email. (I have over ten years of email archives, and in order for me to switch, the new system will need to be able to import and search these archives.)