Originally Posted by Julien Pham
So we need a way to have Calibre handle the AZW3 files with the AZW3 extension. I can add AZW3 to calibre, but then it refuses to send it to my Kindle touch, saying the format is not compatible
Update Preferences -> Plugins -> Device Interface Plugins -> Kindle 2/3/4/Touch -> Customize Plugin -> Select available formats -> azw3.
I'm not sure what happens if you have both a Touch and an earlier Kindle, but selecting azw3 should be safe if you only ever connect a Touch.