Check the inbox of the email account you are sending the books from. If you are using a GMX account from Calibre, login to GMX and see if Amazon has started sending you emails that ask you to click a link to prove you sent the book.
They started doing this a couple of weeks ago because a hacker was able to show that he could send a book with embedded malware from a spoofed email address as described here:
https://threatpost.com/amazon-kindle...-email/163282/
If you don't want to manually authorize every book you send, then switch to a Gmail account. Info on the setup is here:
https://www.mobileread.com/forums/sh...45&postcount=9