Mozilla Thunderbird - Portable Edition is the popular Mozilla Thunderbird email client packaged as a portable app, so you can take your email, address book and account settings with you. You can also get it with GPG and Enigmail preconfigured to encrypt and sign your email.
Mozilla Thunderbird is the safe, fast email client that's easy to use. It has lots of great features including quick message search, customizable views, support for IMAP/POP, RSS support and more. Plus, the portable version leaves no personal information behind on the machine you run it on, so you can take your email and adress book with you wherever you go.