shadows & volumetric material

When I assign volumetric material to an object, I have this kind of shadows:


I have seen that there is a way to have transparent shadows, but…

I have tried everything that came to my mind, but nothing seems to work.

any ideas?

You have to enable “recieve transparent” for the plane in the material buttons

Thank you!

I knew that there was such an option, but it never occurred to me to assign material to the plane.