Which goods and substances am I prohibited from sending abroad?

That varies depending on the carrier. There is a distinction between general prohibited goods and substances, which you cannot send at all, and additional goods and substances that you cannot send to a specific country.

You can find the overview for PostNL here.
You can find the overview for DHL here.
You can find the overview for DPD here (from section 5.2 Excluded from Service).
You can find the overview for UPS here.