I'm not sure why but when I download some epubs and try to send to my kindle it fails. Only after using an online converter to convert them from epub to epub does it then work.
This applies a few fixes for problems with EPUBs that the Send to Kindle service doesn't like.
ebook-convert infilename.epub outfilename.epub
1. like 300 lines of code <https://github.com/innocenat/kindle-epub-fix/blob/master/scr...>
2. runs entirely in the browser and isn't using Calibre scripts or anything else
3. could pretty easily be modified into a downloadable program that runs from both the command-line or something that you can open in your browser