[cybmole]

i suppose the definitive test of what gets forwarded by Amazon would be to bring a copy back from various devices to PC, via USB then inspect the file(s) to determine what flavor of .mobi is contained.
I would not know what to look for, though, & am not that fussed.
For me it was a one off requirement to quickly email a few books to my daughter's new paperwhite, it's not something we plan on doing regularly. Then "quickly" devolved into 2 days of hassle & testing.....
To get stuff to my own KF, I'm good with CC, and with the send to kindle chrome extension for web articles.

I reason that what amazon choose to forward is device independent, as they also add a copy to cloud; & cloud storage setup, is per account , not per device , I think. In my cloud drive folder I see only a generic my send-to-kindle docs, not a specific my send-to-kindle fire hdx docs