Understanding the email setup in respect to calendar events notification

Running nextcloud 15.0.8 via nextcloudpi. Regarding the email setup I notice the following which does not make sense to me:

  1. When I set-up email notifications within the admin’s settings (via smtp) the test email works. So the setup seems ok.
  2. If the email address of the user (also admin) setting up an appointment with attendees is the same as the one used for the email set-up, the notification is sent.
  3. If the email address of the user setting up an appointment with attendees is NOT the same as the one used for the email set-up, the notification does NOT work.

Should nextcloud not simply send the notification with the smtp-settings’ sender email address, no matter what email address a nextcloud user has set-up for his/her account?

Where is my mistake in the train of thought?

Sven