i.Scribe

From Memecode:

Scribe is a small and fast email client with an intergrated contact database and calendar. It supports all the major internet mail protocols and uses international standards where possible. Scribe doesn't required installing or uninstalled and can be used from a removable drive without reconfiguration. It comes with a highly accurate bayesian spam filter and translations to many different languages. Updates are published regularly to respond to problems and to add features. Because Scribe runs on Windows, Linux and BeOS you can take your mail with you when you change operating system. And don't worry about viruses, Scribe protects you from the usual security holes in other email clients with it's own virus safe HTML control and executable attachment lockout.