Mozilla Thunderbird - Portable Edition (formerly Portable Thunderbird) 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.

Features

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.