KDP rejected your Epub. Now what?

Amazon converts your Epub to Kindle's own format when you upload it. If the file is structurally broken, the upload fails or the previewer shows a mangled book, and KDP's error message rarely says what to change. The fix is to find the real errors yourself, locally, before uploading again.

Step 1: validate the file on your own machine

Run the book through the checker. It applies the same schema rules the official Epubcheck validates with, groups the findings by problem, and explains each one in plain language with the source line. It also runs entirely in your browser, which matters here: this is an unpublished manuscript, and it should not be bouncing through a random conversion server. It never leaves your machine.

Check your Epub now

Step 2: fix what it finds

The usual culprits, roughly in order of how often they turn up:

PKG-006: the mimetype entry is missing or not first in the archive. Happens whenever a book is re-zipped by hand. The checker repairs this one automatically.
RSC-005: the catch-all for markup that breaks the content rules, from misplaced attributes to a <div> inside a <p>. The message text tells you which case you have.
RSC-016: an unclosed tag. Fatal, and a single one can take a whole chapter down. Fix the first reported line, then re-check.
RSC-007: a reference to a file that is not in the book. In the Kindle previewer this shows up as blank images or unstyled pages.
NCX-001: the table of contents identifier is out of sync with the package. Also fixed automatically.

Step 3: re-upload a clean file

Once the report shows no errors, upload again and walk through Amazon's previewer. A clean validation removes the structural reasons for rejection. If KDP still complains after that, its message will finally be about something specific and visible (a cover image, a font), which is a fixable conversation rather than a mystery.

Exporting from Word, Google Docs, or an editor like Vellum or Atticus? Validate the export before the first upload, not after the first rejection. Export bugs are common, and catching them locally costs a few seconds.