We report on the investigation of the magnetic damping of a 10 nm thin, poly-crystalline Co25Fe75 film grown by molecular beam epitaxy. Ferromagnetic resonance (FMR) measurements reveal a low intrinsic magnetic damping alpha(FMR)(int) = (1.5+/-0.1) x 10(-3). In contrast, in patterned micrometer wide stripes, spin wave (SW) propagation experiments performed by time resolved scanning magneto-optical Kerr microscopy yield attenuation lengths on the order of 5-8 mu m. From this quantity, we deduce an effective magnetic SW damping alpha(SW,exp)(eff) = (3.9+/-0.3) x 10(-3). For the system studied, this significant difference between both damping parameters is attributed to the non-negligible extrinsic contributions (local inhomogeneities and two-magnon scattering) to the magnetic losses which manifest themselves as a distinct inhomogeneous FMR linewidth broadening. This explanation is supported by micromagnetic simulations. Our findings prove that poly-crystalline Co25Fe75 represents a promising binary 3d transition metal alloy to be employed in magnonic devices with much longer SW attenuation lengths compared to other metallic systems. Published by AIP Publishing.