In March 2015 Amazon augmented their cloud system that requires users to subscribe to Prime in order to store photos and video. If you are not a Prime member, it costs $11.99 per year. When this new system went live Amazon changed their their terms of service for a number of things, which included a new clause that bars 3rd party companies from using “Send to Kindle” or Send to Email”.
The terms now state “You may not charge directly or indirectly to distribute content via the Service. You may not use the Service to send infringing, unauthorized, or otherwise illegal content.”
Baen will continue to sell e-books in the MOBI format and has now posted detailed instructions on how to manually copy them over to your Kindle via USB.