What is the maximum allowed weight of a parcel with an international destination?

That varies depending on the carrier and type of delivery (such as consumer or business). We've compiled them for you by carrier.
 

PostNL
You can send a package weighing up to 23 kg to countries within the European Union. For an XL surcharge of €4.95, the package may weigh between 23 and 30 kg. This option is called 'Large package,' and you can select it under 'Shipping options' when creating a new shipment. For shipments to Belgium, a maximum weight of 23 kg applies.

Additionally, you can send a package weighing up to 20 kg to countries outside the EU. There is no XL size available for packages to countries outside the EU.
 

DHL
A package sent with DHL For You Today to Belgium may weigh a maximum of 23 kg. If you send with DHL For You, DHL Europlus, or Parcel Connect, a package may weigh up to 31.5 kg. However, a surcharge of €4.75 is applied for weights between 23 and 31.5 kg.
 

DPD
For DPD, a package may weigh 20 kg when you drop it off at a DPD location yourself. If you have your package picked up by a DPD Pickup Service, your package may weigh up to 31.5 kg.
 

UPS
A package dropped off at a UPS location may weigh a maximum of 20 kg. When UPS picks up your package with a pickup service, the package may weigh up to 70 kg!