We show that in the presence of a periodic scattering potential the spin relaxation in ultrathin ferromagnets is not a monotonous function of the frequency, as has been usually assumed taking intrinsic Gilbert and extrinsic two-magnon processes into account. The spin relaxation rate is found to substantially increase at characteristic frequencies related to the periodicity of the magnon scattering potential. We propose a theoretical model which is experimentally confirmed in NiFe thin films by artificially introducing different scattering periodicities. As a result, the current general approach for determining spin relaxation parameters in thin films has to be reconsidered.