The imaging performance of an active mm-wave imaging system can be studied using accurate numerical electromagnetic simulations. We present an exact forward solver to calculate the three-dimensional (3D) scattered fields of a two-dimensional (2D) inhomogeneous dielectric object which is illuminated with a given 3D time-harmonic incident field. Since the size of the scattering objects can be very large with respect to the wavelength, a 2.5D configuration is adopted. This reduces the computational cost while it maintains the capability of accurately studying the system performance. The 3D scattered fields are calculated by discretizing a contrast source integral equation with the Method of Moments. The resulting linear system is solved iteratively with a stabilized biconjugate gradient Fast Fourier Transform method.