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 ...

Hi,
I have the same question...
Last Saturday I filled a FP but any ATC received it.

There is no such things as formal ATC acceptance or rejection of VFR flightplans across Europe. They are simply messages that are sent to a tower/ARO unit.
If a recipient denies to you that they have received a flightplan, get in touch with us, because we can always prove that they did. What they do with it after receipt is of course beyond our control.
If a recipient replies to the flightplan message, it will in most cases be forwarded to you by email automatically, provided they respond in a standard format.

Tim, when I submit a flight plan via, e.g., Austrocontrol's Homebriefing (www.homebriefing.com) website I can specify whether I want to receive further filing related messages via email or sms. I get "FPL ACCEPTED" messages for each flight plan. I can, of course, also check on the status of each filed flight plan via the Homebriefing website.

-- Chris.

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.