What happens if the delivery location is a solid block?

If the delivery location isn't suitable (ex. it's inside a solid block, in a liquid, or floating), nearby Y coordinates will be scanned until a suitable one is found. If there are no suitable Y coordinates at the given X and Z position, the package will be marked as undeliverable and returned to the sender.

For more detail, check out EntityEnderMailman::findValidDeliveryHeight.