A computer simulation of the transmission of 20 MeV electrons through a (001) MgO single crystal was performed. The calculation model represents a combination of the usual Monte Carlo simulation of projectile transmission through a heterogeneous absorber and the binary collision model of trajectory tracing. The dependence on the penetration depth of electrons moving within the limits of separate Seitz unit cells was obtained. The influence of electron-electron scattering for such projectiles was studied in comparison with that of thermal lattice vibrations. The boundaries of the occupied phase space of transverse energies and angular momenta for the swift electrons motion along the atomic strings were determined.