  In this paper, we design robust and efficient linear solvers for the
numerical approximation of solutions to Maxwell's equations with dissipative
boundary conditions. We consider a structure-preserving finite-element
approximation with standard Nedelec--Raviart--Thomas elements in space and a
Crank--Nicolson scheme in time to approximate the electric and magnetic fields.
  We focus on two types of block preconditioners. The first type is based on
the well-posedness results of the discrete problem. The second uses an exact
block factorization of the linear system, for which the structure-preserving
discretization yields sparse Schur complements. We prove robustness and
optimality of these block preconditioners, and provide supporting numerical
tests.
