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 the future.
I never investigated this deeper but I have similar issue on my Nook Glowlight. Despite it supporting ePUB files some of them just throw an error on the devices. Converting them ePUB to ePUB solves the problem.
I've learned recently at my new job that ePubs are just a giant zip file of html, css, etc. Sometimes you need to rezip them if they are not working quite right and I imagine that is what converting them from ePub to ePub is doing.
Are they going to ship a software update? Would be a shame if that service doesn't work any more in the future.