  In this paper we prove the propagation of singularities for the wave equation
on differential forms with natural (i.e. relative or absolute) boundary
conditions on Lorentzian manifolds with corners, which in particular includes a
formulation of Maxwell's equations. These results are analogous to those
obtained by the author for the scalar wave equation and for the wave equation
on systems with Dirichlet or Neumann boundary conditions earlier. The main
novelty is thus the presence of natural boundary conditions, which effectively
make the problem non-scalar, even `to leading order', at corners of codimension
at least 2.
