Amazon’s Kindle Direct Publishing is scrapping the MOBI format for ebooks.
From June 28, 2021, KDP says it is ‘making it simpler for authors to publish ebooks on Kindle’ and will no longer support files in MOBI, PRC or AZK formats when publishing new reflowable eBooks or updating the content for previously published ebooks.
MOBI was KDP’s preferred format for several years but KDP has been recommending EPUB over MOBI for some time now as it has been moving away from MOBI gradually following criticism over the years for using the proprietary format.
KDP is now asking for EPUB, KPF (Kindle Create files), or DOC/DOCX (Microsoft Word files) for reflowable ebooks. MOBI files will still be accepted for fixed-layout ebooks.
You don’t need to take any action for reflowable ebooks already published unless you are updating the ebook files.
It’s worth noting that, at present, if you send an ebook document to your own Kindle, for example to check over a work in progress, then it will still need to be in mobi format as the Kindle verify process will reject an epub document.