1. From Thunderbird/Evolution/gmail/wherever: Save message (as .eml).

2. $ sudo apt-get install mpack # or equivalent for your OS - install package
called "mpack"

3. $ munpack message.eml
# bash, multiple files:

$ for f in *.eml; do munpack $f; done Orthunderbird :
- right click on the file -> open with -> open with other applications…

- open the textfield cliking on use a custom command and type :
thunderbird %f and then click on Open button.