Rated 5 out of 5 stars

Seems to work fine (TB 10.0.1). You should probably explain a little more; for example that one has to add your button to the Write window's toolbar, enable the Contacts sidebar, select the list from there and click "Add to Bcc" (for example), then click your button.
I even had nested lists (one list within the list I chose) and clicking the button twice expanded the inner list.
Would it be possible for you to add a button to delete the sender's entry from the recipients (if present) in case it's a Forwarded message (and you don't want to send it back to him)? Would be perfect!
Thanks for this!

This review is for a previous version of the add-on (0.2). 

Thanks for the feedback - unfortunately I don't have much spare time ATM to implement any changes...
But I think to remember an add-on called "Not to" to enable sending a message to a list except to some named addresses - HTH ;)