I have problems with mailbox.org as described above.
In the mailbox-forum Peer Heinlein (AFAIK: CEO) pointed to greylisting.
If spamgourmet changes its behaviour on delivery, the problem will disappear.
You can find his answer - and the full discussion - here:
https://userforum.mailbox.org/topic/647 ... ment-28930
Can spamgourmet's behavior be changed accordingly?
Regards
HD