Also send-to-kindle today rejected a mobi I sent by email with:

>>>

Send to Kindle supports the following document formats: Adobe PDF (.pdf) Microsoft Word (.doc, .docx) Rich Text Format (.rtf) HTML (.htm, .html) Text (.txt) documents Archived documents (zip , x-zip) and compressed archived documents MOBI (.azw, .mobi) (will not support the most up-to-date Kindle features for documents) JPEG (.jpg), GIF (.gif), Bitmap (.bmp), and PNG (.png) images.

Additionally, Send to Kindle emails now supports EPUB (.epub). Later this year, we’ll also be adding EPUB support to the free Kindle app for iOS and Android devices and the Send to Kindle desktop app for PC and Mac.