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See also: Why does Jutoh warn that multiple configurations use the same Epub file name?
You may see this message:
TIP (Jutoh): the Epub file for the configuration Kindle should be used for Kindle only.
You have checked both 'Generate Epub' and 'Generate Kindle' in your configuration.
You can upload the Epub file generated with this configuration to Amazon KDP and view it with Kindle Previewer, but you should not upload it to other distributors since it contains Kindle optimizations. Use a separate configuration for generating Epub for non-Kindle purposes.
This is because if your Kindle configuration is also generating an Epub file, it will add Kindle-specific optimizations that may cause problems or rejection if you accidentally upload this Epub to a non-Amazon distributor. So you should use the Epub configuration for Epub configurations, and Kindle configuration for Kindle files, and ensure that you use the correct Epub file.
There are several reasons why you might have done this:
you may have seen a message in Kindle Previewer 3 that told you to use Epub files instead of Mobi files;
you may wish to generate Mobi files for viewing in Kindle for PC/Mac, sideloading to a Kindle device, uploading to your web site, or sending to a non-Kindle distributor;
you may have done this after pressing Check for your Kindle configuration and receiving a message that suggested you switch on Generate Epub; or
you may have assumed that you need to edit the Kindle configuration in order to generate normal Epub files, instead of choosing the Epub configuration.
In case (1), Kindle Previewer and KDP are both capable of handling Mobi files, but Amazon is trying to discourage it. So, you can use Epub if you prefer and wish to suppress the Kindle Previewer message.
In case (2), it's fair enough to generate Mobi files for purposes other than Kindle Previewer and KDP.
In case (3), when you press Check, you are asking Jutoh to run EpubCheck, a 3rd party tool that checks Epubs. EpubCheck cannot run on a Kindle file - only an Epub file. Therefore, you need to generate an Epub for EpubCheck to work. But generally it's best to use the Epub configuration, not the Kindle configuration, to check the generated book file because Kindle optimizations may interfere with EpubCheck.
In case (4), please use the "Epub" configuration to generate Epub files to send to Epub-based distributors.
If you really want to create Epub files for Kindle configurations, you can ensure that the Epub file names are different (via Book file name) to avoid file name clashes. By default, the Mobi and Epub file names for a Kindle configuration are suffixed with _Kindle.
You can switch off the Jutoh tip via Advanced Settings by clearing Multiple formats warning.
From 2020, Amazon has started to insist that people use the Epub format to preview and upload their books. This is despite the fact that the Mobi file format (extension .mobi) still works perfectly well in Kindle Previewer 3 and when uploading to KDP, and despite the fact that Jutoh is using Kindlegen as found within the Kindle Previewer 3 software itself to generate .mobi files. So when Kindle Previewer 3 pops up a little message to warn you off using .mobi files, it's effectively chastising you for using part of Kindle Previewer 3.
It's slightly inconvenient because if you're also creating Epubs for other distributors, it's easy to get confused between the Kindle-optimized Epub, and the non-Kindle-optimized Epub. By default, Jutoh generates them with different names (for example Book_Epub.epub, Book_Kindle.mobi, Book_Kindle.epub), so please be careful to use the correct file for the correct purpose if you have checked Generate Epub within a Kindle configuration.
You may well also want to upload .mobi file to your web site or to a Kindle device, or simply to view it in Kindle for PC/Mac, which is a much more convenient viewer than Kindle Previewer if you just want to read the content as opposed to check the formatting. So you'll probably end up generating both Epub and Mobi with your Kindle configuration.
If you want to completely bypass Mobi generation, you can do this by going to the Helpers page in the Preferences dialog, and setting Kindle file generation method to None (Kindle-optimized Epub only). Now, your Kindle configurations will generate Epub files only - the Kindlegen tool won't be used. Why not just use an Epub configuration instead? Because you need to check Generate Kindle for Jutoh to generate Kindle optimizations.
Contents | Start | End | Previous: KB0146: Why do hyperlinks (URLs) not work in Kindle fixed format books? | Next: KB0148: Why are numbers smaller than other letters in Kindle?