.MOBI files can no longer be sent to Kindle since the beginning of 2023 #526
Description
According to Amazon (see this page):
But the current implementation of the function GetMostInterestingDataToSendToKindle() in the lib/Book.php file, the .MOBI file is the preferred one for sending by email. And sending by email is a very popular way to get an ebook on your Kindle!
It seems necessary to change this, for example by preferring the .EPUB format, or at least by removing the .MOBI file from the list:
$bestFormatForKindle = array('PDF', 'AZW3', 'EPUB');