Accepting meeting request from Outlook or Gmail does not seem to work

While using the synchronization of Nextcloud Calendar with eM Client we tested these two flows:

  1. adding an event ourselves in eM Client asking third parties to join > this worked well

  2. accepting an event created by a third party (via outlook or gmail) > this does not work: the third party does not receive a notification that we accepted the event

I guess we are not the first with this issue? How to solve it? Didn’t find the wright instructions in this forum yet.