Amazon has informed all Kindle users via email that starting in August 2022, it will no longer be possible to send MOBI (.mobi, .azw) files to your library due to MOBI being an older file format that doesn’t support the latest Kindle features for documents. Additionally, they have introduced support for EPUB (.epub) files, which can be sent to your library.
Which file format does Push to Kindle utilize, and does this change have any impact on it?"
Hi zain, Push to Kindle switched to EPUB sending some time back, so this change will not affect Push to Kindle users. This is from our blog post on May 30, 2022
Amazon EPUB delivery
As discussed in our previous post, Amazon has announced that its Personal Documents Service will not accept MOBI files in the future and instead has started officially accepting EPUB files. As such Push to Kindle is now sending EPUB and not MOBI files. The end result should be the same as before.