Ohhh ... I was wondering that some mails did not reach me any more. And I took a look into my SpamGeek logs.
Over time the SPAM score of the domain got higher and now it was blocking also other senders from that domain.
It got higher because the words I added to the negative list. And the ratio between good and bad mail changed..
This is a companion discussion topic for the original entry at https://blog.nashcom.de/nashcomblog.nsf/dx/spamgeek-works-o365-finally-got-blocked-sending-too-much-spam.htm