As of August 2022, Amazon is discontinuing the ability to send MOBI files to Kindle via the Amazon Kindle Personal Documents Service that is used by Send to Kindle.

To learn more, see these articles:

So, the important takeaway is the following:

As of August 2022, the only way that a Leanpub ebook will be able to get on to an Amazon Kindle is by sending it to the Kindle as an EPUB file.

So, since August is fast approaching...

Leanpub is discontinuing MOBI support on July 25, 2022. Leanpub readers can now send Leanpub EPUB files to Kindle in a similar way that Leanpub MOBI files used to be sent to Kindle.

Luckily, Leanpub has been making EPUB files for over a decade! Actually, the way that we have been making MOBI files is by using our EPUB files as input. So, now we just don't need to bother making MOBI files anymore.

If you bought a Leanpub ebook which was generated on Leanpub, we already produced PDF, EPUB and MOBI. Since the MOBI files are not helpful anymore, we are no longer showing them in your libraries. Instead, just download the EPUB and send the EPUB to Kindle, or use our Send to Kindle feature from within your Leanpub library to send the EPUB file the way that you would have sent the MOBI file...

To be clear:

The process that you follow to do Send to Kindle from your Leanpub library is UNCHANGED. We just send the EPUB file now, and Amazon converts it for you automatically.

If you are a Leanpub author who has put a lot of work into testing our MOBI files, chances are this was not wasted: presumably, Amazon's process for doing Send to Kindle with EPUB will benefit from the QA work that you did ensuring that our MOBI files worked nicely...