I have some mobis that I can't get to appear on my Kindle (see
here). I've changed my preferred output in Preferences - Behaviour - Preferred output format to AZW3. However, when I send my books to the Kindle they still appear on it as mobi files. Do files need to be converted in advance? I was expecting this option to make Calibre convert the books to AZW3 upon the 'send'?