In a recent communication, Amazon has alerted Kindle users about significant changes set to take effect from next month. The notification pertains to the phasing out of support for sending MOBI (.mobi, .azw, .prc) files through the “Send to Kindle” feature, starting November 1, 2023. This change, as News18 pointed out, specifically impacts users attempting to send MOBI files via email and Kindle apps on iOS, Android, Windows, and Mac.
So just set Calibre to convert the books to mobi before sending it to them
That doesn’t work for the workflow of sending articles to my Kindle with a bookmarklet.