Being a German email address is not the issue. The issue is that somewhere in its travels it appears that the sending SMTP server was retransmitted by another SMTP server so the sending SMTP server indicated in the outer wrapper is different from the source SMTP server in the message header. Messages are often forwarded through two or more SMTP servers so their may be multiple transmission layers each referencing a different forwarding server. Spend some time looking at the RAW headers and you can get dizzy trying to trace the route taken from server to server.
In this case it would appear that the receiving POP/IMAP server in Germany found one of the links suspect and their policy (implementation) of the tightened EU and German regulations, rejected the message. The policy under which the message was rejected is unique to the receiving POP/IMAP server so particularly in light of the new rules there is a possibility of legitimate traffic being rejected.
Therefore the problem is not in your computer it is most likely in the receiving POP/IMAP server's policy mechanism or it could be a failure on the part of some SMTP server in the chain to notify the internet of a new or different server to server linkage. This will get fixed, but in the meantime you may have to use another email provider to send messages to this address.
I have seen a similar case recently on another forum so yours is a rare but not unique situation. This is reminiscent of a period probably 30 or more years ago when SPAM was first becoming a problem and the U. S. government put the onus on the ISPs to control the explosion of SPAM that was estimated to be eating up to 60 0r 70% of internet bandwidth of the time. It was relatively easy to get your personal email address on a list of ill-behaved email senders and could take days to get it removed from the list or longer for it to age off of the lists. I got on some lists because I did a Reply All to a message from an organization that had over 100 addressees.
