Amazon is not specifically blocking emails from Calibre -- they have no idea if an emailed book comes from Calibre or not. Amazon is requiring a separate authorization step for every book emailed from a non-authenticated server such as GMX, because a hacker recently showed that you could do "bad things" by sending a book with embedded malware from a spoofed email address. There should be authorization requests for each book in your GMX inbox.
You can send books without extra auths from a Gmail account or some other email service that uses the proper headers. A few days ago I posted some notes on how to set up Calibre to use Gmail here:
https://www.mobileread.com/forums/sh...45&postcount=9