GPG

will give you all you need:

https://portableapps.com/support/thunderbird_portable#encryption

try here, enigma and gpg is impleneted and works fine as portable.

you can try this easy-to-use portable-application at http://gpg4usb.cpunk.de

It's a simple texteditor combined with a GnuPG-frontend. Running from an usb-pendrive on linux- and windows-machines.