Beginning in late 2022, you’ll no longer be able to send MOBI (.AZW, .MOBI) files to your library using Send to Kindle. This change won’t affect any MOBI files already in your Kindle library. MOBI is an older file format and won’t support the newest Kindle features for documents.
For years, Amazon has used the Mobipocket format (.mobi) rather than the ePub format for Kindle books. Recently it developed its own format, KF8 (and now KFX), and has been using that instead. Everyone else uses ePub. Publishers sell books direct as ePubs, but are forced to include .mobi versions for compatibility reasons.
But this is changing. Amazon announced that later this year, its email and drag-and-drop send tools will accept ePub format files. Amazon will then convert those files to KF8 and deliver them to Kindles, but at least the user won’t have to do the job themselves. Progress.
—Linked by Jason Snell