In this paper the quantum vacuum energies induced by massive fluctuations of one real scalar field on a configuration of two partially transparent plates are analyzed. The physical properties of the infinitely thin plates are characterized by two Dirac-$\ensuremath{\delta}$ potentials. We find that an attractive or repulsive Casimir force arises between the plates when the weights of the $\ensuremath{\delta}$'s have equal or different sign. If some of the plates absorb fluctuations below some threshold of energy (the corresponding weight is negative), there is the need to set a minimum mass to the scalar field fluctuations to preserve unitarity in the corresponding quantum field theory. Two repulsive $\ensuremath{\delta}$ interactions are compatible with massless fluctuations. The effect of Dirichlet boundary conditions at the endpoints of the interval $(\ensuremath{-}a,a)$ on a massless scalar quantum field theory defined on this interval is tantamount to letting the weights of the repulsive $\ensuremath{\delta}$ interactions go to $+\ensuremath{\infty}$.