This is the software publisher’s description.

Mozilla Thunderbird, Portable Edition is the popular Mozilla Thunderbird email client bundled with a PortableApps.com launcher as a portable app, so you can take your email, address book and account settings with you. You can also add in GPG and Enigmail 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 address book with you wherever you go.