We study the transmitter optimization for the flat multi-input multi-output (MIMO) channel under nonlinear distortion from the digital-to-analog converters (DACs). Our design is based on a minimum mean square error (MMSE) approach, taking into account the effects of the transmitter nonlinearities. Our derivation does not make use of the assumption of uncorrelated white distortion (quantization) errors and considers the correlations of the quantization error with the other signals of the system. Through simulation, we compare the new optimized linear transmitter to previously proposed linear transmitter designs when operating under DACs in terms of uncoded BER.