"KMail has long been my Linux email client of choice for a
number of reasons: nice clean interface, easily customizable and
configurable, stable, and more features than you can shake a stick
at. Today we'll dig into migrating from other email clients,
encrypting messages and key signing, and configuring multiple
accounts and identities...
"You should also know that KMail requires kdelibs, kdebase, and
kdepim. It runs on Linux or most any Unix. The latest stable
version is 1.7; this article covers versions 1.5 and up..."