I looked through the whispers, there definitely wasn’t an abuse by someone else. I was sending about 10 messages 2-3 times a week (only to users who opted in voluntary) and each message clearly states how to unsubscribe from the feature so I don’t think it received much spam reports.
But I realize that sending the same message to several people at the same time over and over again can easily be interpreted as spam.
Thanks a lot for the reply, I will look into that.