Kindle Send-to-Email Not Working

[Bad Astronaut]

I used to use send-to-email all the time to send documents from my computer to my kindle. Lately it hasn't been working.

As a test I took a text file, converted it to .mobi in Calibre and sent it through email (also through Calibre). The document I sent is attached (taken from my gmail "Sent Items" folder to be sure). The email I got back from Amazon is below. Any ideas on what's going wrong?

Dear Customer,

The following document, sent at 09:31 AM on Fri, Dec 06, 2013 GMT could not be delivered to the Kindle you specified:
* Test Doc - Unknown.mobi









The Kindle Personal Document Service can convert and deliver the following types of documents:
Microsoft Word (.doc, .docx)
Rich Text Format (.rtf)
HTML (.htm, .html)
Text (.txt) documents
Archived documents (zip , x-zip) and compressed archived documents
Mobi book

Images that are of type JPEGs (.jpg), GIFs (.gif), Bitmaps (.bmp), and PNG images (.png).
Adobe PDF (.pdf) documents are delivered without conversion to Kindle DX, Second Generation and Latest Generation Kindles.

Adobe PDF (.pdf) can be converted to Kindle format and delivered on an experimental basis.

If the document that failed belonged to one of the above document types, please ensure the document is not password protected or encrypted. Note that the Latest Generation Kindles support password protected PDFs.

Helpful tips for personal document attachments:
•The file size of each attached personal document should be less than 50MB (before compression in a ZIP file)
•The email submitted should not contain more than 25 attached personal documents

You can learn more about transferring personal documents to your Kindle in our help pages:
http://www.amazon.com/kindlepersonaldocuments/

If you would like immediate assistance, please contact customer support at +1-866-321-8851 (US customers) or +1-206-266-0927 (International Customers).

Sincerely,
Amazon Kindle Support

Please Note: This e-mail was sent from a notification-only address that cannot accept incoming e-mail. Please do not reply to this message.

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If you have further questions about Kindle, please visit the Amazon Kindle Help pages
The Help pages will provide you with answers to common questions and provide you with a direct link to Amazon Kindle specialists.
To contact Amazon Kindle Customer Service, simply click a button in the Contact Us box on the Help pages to receive assistance by e-mail or phone.

Reading your personal documents on Kindle is now easier than ever. Learn more about "Send to Kindle" at www.amazon.com/sendtokindle

Update: I tried sending a raw text file and it worked. Could Amazon be rejecting Mobi files created by Calibre?

[Bad Astronaut]

I figured it out - my MOBI conversions were set to use "both" (old and new) formats. After setting this to use the old format and re-converting I can email documents to my Kindle successfully.

Still I'm curious if this is a Calibre bug. I know the new format is more flexible in some ways so it would be nice to be able to use it.

[kovidgoyal]

amazon does not allow email delivery of KF8 documents.

[eschwartz]

If you can, you may want to just send your books directly via usb to your kindle, to get the better flexibility of KF8.

The only downside is it won't be sync'ed between devices.