The domains that do enforce DMARC are apparently configured so badly that the German secure email provider mailbox.org decided not to honor DMARC.
See this thread in German: https://userforum.mailbox.org/topic/10676-mailbox-org-akzept...
See this thread in German: https://userforum.mailbox.org/topic/10676-mailbox-org-akzept...
The message you obviously refer to as just an educated guess by a forum user who seems to be experienced in email topics. It could be that the guess is correct. It could be hat the consumtants are too overloaded to configure things differently. Another user has that guess. We don't know as long as the provider does not answer.