I like kmail. It is the default mail client for KDE desktops. I have used Thunderbird for a long time now, but kmail is equivalent. So is evolution, which I use as well. Ultimately they all do the same thing; they are mail clients. Each one has comparable features, although they all look slightly different. I tend to use Thunderbird, but have grown an affinity for kmail. In particular, when Using Gnome I use Thunderbird, and when using KDE I use kmail.