This might be quite a newbie question, but anyway: When filing a flightplan out of skydemon I receive an email "that your flightplan has been successfully filed". But how do I know it is accepted or rejected by the ATC unit? Thanks for your answers ...

That is a feature specific to the combination of Austria/Homebriefing. It's certainly not an international standard and as far as we're aware is not used outside of that combination. If Austrocontrol send accepted or rejected messages back to other flightplan sources such as ourselves, they will of course be communicated to the subscriber who filed them.