I usually send books to my device and my husband's device with the Content Server. I use a gmail proxy, and the email address is an approved sender to both. But every 10 books or so, I get the following email
Dear Customer,
The following document, sent at 10:00 PM on Wed, Dec 03, 2014 GMT could not be delivered to the Kindle you specified:
* xxxxxxxxxxx.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
The book did not exceed the size limitation. I usually just resend it, or sideload it. Does anyone else get these error messages? Do you know what causes it? Is it Amazon is looking at every 10 emails? I don't send any more than 2 or 3 books at a time, every 5 minutes.