Superconductor-Ferromagnet (SF) heterostructures are of interest due to numerous phenomena related to the spin-dependent interaction of Cooper pairs with the magnetization. Here we address the effects of a magnetic insulator on the density of states of a superconductor based on a recently developed boundary condition for strongly spin-dependent interfaces. We show that the boundary to a magnetic insulator has a similar effect like the presence of magnetic impurities. In particular we find that the impurity effects of strongly scattering localized spins leading to the formation of Shiba bands can be mapped onto the boundary problem.