Okay so this is weird.. I sent a mobi file and it didn't work.
Dear Customer,
A document sent at 10:24 AM on Wed, Dec 13, 2023 GMT could not be delivered.
Send to Kindle can convert and deliver the following types of documents: • EPUB (.epub)
• Adobe PDF (.pdf)
• Microsoft Word (.doc, .docx)
• Rich Text Format (.rtf)
• HTML (.htm, .html)
• Text (.txt) documents
• Compressed documents (.zip, .xzip)
• JPEG (.jpg), GIF (.gif), Bitmap (.bmp) and PNG (.png) images.
Yeah, it's silly and odd and likely done to push customers towards formats that they have greater control over.
Those epubs that aren't really epubs, randomly disallowing azw3 files (that they support officially!!!) from being downloaded directly from the kindle's built in browser and other restrictive behaviour are part of this. That's why I'm eventually looking to enable epubs on kindle once the people at mobileread find a way to do it. Apparently calibre can be set up to send files too via email so that's another option.
Oh, I assumed they would just be supporting it as standard now, that's annoying. I've just gotten myself an Oasis so.. well, guess I'll rustle up my mobi files once more.