Hello @Ulrich Winter ,
While sending event invite to attendees, please check the "Request Responses" option. If this option is unchecked, you will not be able to receive the accept/decline responses from the attendees.
Please refer the attached screenshot:
You can refer to this similar post for more information: https://learn.microsoft.com/en-us/answers/questions/1161430/how-to-track-response-from-other-email-services-li#comment-1168123
Hope this helps.
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