I just tested my GMX send and it is working with 6.6.0/6.6.1.
Perhaps you might want to pay special attention to the 2 lines I've copied below:
Code:
reply: b'535 Authentication credentials invalid\r\n'
reply: retcode (535); Msg: b'Authentication credentials invalid'
They are saying that your GMX credentials are not valid. Can you log in at gmx.com using the credentials that you are trying to use with calibre?