Thanks Dorothee for letting me know. I also had another report of a mail that was unable to be delivered to this census email adress.
However I also tested it tonight, and there was no problems.
Again, I am on to support for help. What it is about is not that the email is not working, but that somehow somewhere along the lines (which I am not sure completely of) but the recieving mailbox (in your case web.de) rejects what it thinks to be a mail from an insecure connection. This may not at all be the case (that it is insecure) – but just thought to be, and its all a precaution from for example web.de.
This is what I am trying to find out via support.