The email-based implementation of send-to-kindle limits files to 25mb, which I've found to be prohibitive.
Amazon has an API to send larger files to kindle devices, for example via their
Send to Kindle for Mac product. I've created an open-source python client for the API here:
https://github.com/maxdjohnson/stkclient#readme
Would you be interested in integrating it into Calibre as an alternative or replacement for the email-based approach?
I've explored implementing it as a plugin, but I wasn't able to have the upload trigger automatically after news is downloaded. In any case, I figure it would be better as a core feature, as the email based implementation is today.