.We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of e^{+}e^{-} pairs above the pi^{0} mass in the exclusive channel dp rightarrow npe^{+}e^{-}(p_{spect}) as compared to the exclusive channel ppe^{+}e^{-} measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double- Delta excitation combined with intermediate rho-meson production. Differential cross sections as a function of the e^{+}e^{-} invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical sides.