We report on an experimental study of the mechanisms of stimulated emission from cavity polariton states of CdTe-based quantum well microcavities. We show that the nonlinear threshold-like behavior of the photoluminescence unambiguously occurs in the strong coupling regime and that the stimulation mechanism is not a usual laser effect obtained by population inversion of an electron-hole plasma or of localized exciton states. The experimental observation of laser-like emission can be explained by condensation of polaritons on radiative states as a result of boson statistics mediated by exciton-exciton scattering processes.