Same goes for Belgium. Proof of sending is enough and legaly binding. This is done by sending it via registerd mai, so it will cost you. Extra price is about 5 EUR. The receiver even needs to sign for it and vecause everybody in Belgium has an ID card, only the recipient will be able to sign it off. For a bit more, you can even get the proof back that the person signed for it.
Two things: The recipient does not (want to) sign. No problem. You send it and that is all there is.
The person does not have a forwarding adress: No problem. Financial institutes and the law are able to check any new adress via several means, while still guarantee to the individuals privacy.
As long as a reasonable amount of search for a new adress can be proven, it is all well for the sender. In general this means checking at e.g. the NBB.
If it is a about a trial, you can not use the defence "I did not know it was going on.". Instead you will most likely agree with everything that has been said, as you (or your representation) is not there to deny it.
If you try the aproach of "But I wasn't home during that 14 day period when I could pick it up." then you MIGHT be able to get a re-trial if you are able to proof that it is true and even that can be denied if you have done the same before the trial.
OTOH you might also piss off the judge who thinks you are wasting the courts time and you do NOT want to piss them off.