How to send an EPUB to Kindle

Quick answer

EPUB is on the supported list in Amazon's Send to Kindle help page, so most files go straight through. When one fails, the email back often mentions E999. Readers who hit it a lot have a short list of fixes.

What 125x is: 125x is a web app at 125x.app. You connect your YouTube subscriptions, and it gives you a feed of the latest videos from the channels you follow, each with a summary: a verdict, the 5 key takeaways, and the spoiler, plus Actionable and Lessons tabs with more tips. You read that and decide what's actually worth opening.

Open 125x

Sending it

Web: open Send to Kindle in a browser and drop the file in.
Email: attach it and send it to your Send to Kindle address.
Phone: share the file to the Kindle app.

Fixes readers use for E999

Open the book in Calibre and make sure the language field is set, like English. A missing or odd language tag is the most common cause readers report.
Run Calibre's Check book and fix what it flags.
Convert the EPUB to EPUB again in Calibre, which rebuilds the file.
Shrink very large files with lots of images.
Try a different send method. Some files fail by email and go through on the web.

Where 125x fits

On 125x, every video in your subscriptions feed comes with a summary: a verdict, 5 takeaways and the spoiler, plus an Actionable tab with steps and a Lessons tab with the bigger ideas.

FAQ: questions people ask

What does E999 mean on Send to Kindle?

It's a general delivery failure. Readers often fix it by setting the book's language in Calibre.

Does Kindle support EPUB?

Yes, through Send to Kindle, which converts it for your library.

Why won't my EPUB send to Kindle?

Common causes are a missing language tag, a damaged file, or a file that's too large.