we are doing more tresting. some file got through but appear as docs not as books.
google says change [PDOC] to [EBOK} oon mobi output page. there s no mention of [EBOK] is hte calibre pop up help ?
also we know that mobi: old is accepted by the kindle email and that , MObi new is rejected.
mobi: both seems also to be accepted but I wonder if amazon just throw away half of that & thus it would be more efficient to not bother sending it. the file size for mobi: Both is 50 - 100% larger. e.g. a sample mobi

ld book 2MB, but mobi:both or same book 3MB
I guess the easiest way to test that is to email mobi: both to my own kindle fire and then check file size sent vs file size received, unless someone already knows the answer.
my daughter's kindle is an e-ink paper white 2nd gen , so what she gets via the email route may be different to what my kindle fire gets ?