[cybmole]

I spent 30+ minutes on phone with amazon re this. their front line CS are clueless as amazon have not documented what happens. I ended up emailing them with the explanation after lots of testing. My thread is in calibre - conversions subforum.

to reiterate: you can convert to one of 3 options: mobi: old, mobi: both, mobi: new.
but when you email that output file to send-to-kindle address - "new" is stripped out. so if you email mobi: both then only "old" gets through., if you email mobi: new only, you get an error email.
Also, you cannot email azw(3) to kindle address as it is not an allowed extension.

when you email "both" you don't get to "fool" amazon you just get to waste time & bandwidth by creating an email of the large mobi: both file, only for 1/2 of it to be discarded en route, check file size sent & compare with file size on device.

PS one advantage of the email route is that it also places a copy in your amazon cloud folders, though you can do that yourself if you have cloud folder + app on your PC

there are other methods which will put mobi: new aka azw on your device e.g. via usb or caiibre companion if it's a tablet.

there's lots more fun stuff explained on other threads which address why does my book appear as a document, and why is my cover not displayed correctly on carousel. all are fixable.