exactly what I was looking for Rated 5 out of 5 stars

It allowed me to tag my emails instead of putting them into folders.
It is possible to assign many tags to the selected message (which is great!!!), whereas the message might be put into one folder only.
The functionality I'm missing: to filter the tagged messages easily. The only way I know is to search for the tagged messages.
I would prefer (for instance), if there is a separate panel, that shows all the tags, that the selected messages are tagged with (apart from the toolbar that presents all available tags).
By clicking the tags, one can easily narrow the selection and find the messages that he is interested in.
I miss that functionality a lot, so I will try to implement it myself, although being not a professional programmer neither experienced with TB, I might need some time :(.

This review is for a previous version of the add-on (0.8.12).