I'm extremely confused about this. I have an older Kindle Paperwhite (PW 3/ 7th gen I think?) and when I used the 'email sent' feature I got that mail about dropping mobi support in favor of epub, but when sending an actual epub I got an error for sending a wrong file, ostensibly because the Kindle only supports mobi. Are they going to ship a software update? Would be a shame if that service doesn't work any more in…
Probably because the EPUB file has some problem. Amazon is very strict about EPUB validity.
They're also bad at converting perfectly valid epubs to their own format.