Send-to-Kindle fails on large Mastering Cardano EPUB (~134MB, E999 error)

[akonior]

I tried sending the generated mastering-cardano-v1.0.0.epub to my Kindle using the official Send to Kindle feature. The file size is about 134MB.

The delivery consistently fails with the following message:

Dear customer,
The following document(s), sent at 07:16 AM on Tue, Aug 26, 2025 GMT, could not be delivered due to E999 - Send to Kindle Internal Error:
mastering-cardano-v1.0.0.epub
We apologize for the inconvenience.

Observations:

• According to Amazon, Send to Kindle supports EPUB files up to 200 MB.
• However, this EPUB (~134 MB) always fails with error E999.
• When I manually reduced the images inside the EPUB so that the final file was <50 MB, and run github action epub build on a fork: the upload succeeded without any issues.
• This suggests there might be a practical file size (or image size) limitation below the officially stated 200 MB.

Proposed Solution / Workaround:

• Consider reducing the resolution/size of images embedded in the EPUB so that the final output is <50 MB.
• Alternatively, test with intermediate sizes (e.g. 50–100 MB) to determine the maximum reliably accepted size for Send to Kindle.

This would make it possible to read the book on Kindle without manual post-processing.

[Profsmith9000]

@akonior When using the Send to Kindle feature, did you send to email or did you try to send via the web app or one of the other apps?

The Send to Kindle email feature has a limit of 50MB, while the web app should support 200MB

[Profsmith9000]

I was able to get it to fail with the Web option, though my EPUB is ~98MB

[akonior]

I used the web app https://www.amazon.com/sendtokindle. The EPUB file is too large to send by email.

[scott-kl]

I have the same problem. Additionally, I converted the epub file to an azw3 file using calibre, and then manually copied that into the kindle's documents folder via usb cable. This worked, in that the book showed up in my library, but as soon as I scrolled to the first image in the file (section 1.3) the kindle locked up and crashed, forcing a restart. Navigating by links can take me anywhere in the book so long as that page doesn't have an image on it, if it does, it crashes. So this looks like a memory problem.