Need help for a PM thing i want to do between groups

[Goodman_2020]

Hello

I would like to prevent registered users (members) to contact each other via PM or email but without setting to "0" the private message box (you'll understand why just after) but i would like that this primary group can contact and be contacted by a secondary group i'll create, and can send each others PM. (so you understand why i need that they keep the capacity of their box in case they are contacted by a member of a secondary group).

Is it possible? if yes, how to do it? is there a hack for that?

Thx