We present the results of a systematic study of the temperature-dependent resistivity and of T-c of a single crystal MgB2 sample as a function of point defect concentration. We have found linear, albeit weak, correlation between the decreasing superconducting critical temperature and the residual resistivity and no variation of the slope of the rho(T) curve at high temperature. These findings reinforce the already existing picture of s-wave pairing for the superconductivity. The interband scattering is low despite increasing disorder. Somewhat surprisingly, Matthiessen's rule is followed even in this two-band metal.