I am struggling with proving the complete positivity of a general map ( granted it is CP ). The reduction map is defined as $$ \rho \rightarrow \mathrm{Tr}(\rho)I - \rho $$ It is a trivial job to show the positivity of this map. I don't know whether this map is CP at first place.
Is there some physically intuitive argument which explains why this map isn't (or is) completely positive (such as an entangled state on which it acts in a non-positive way)? (Which is something which is not discussed in the answers to the post marked as a duplicate on math.SE.)