  We extend the framework of a posteriori error estimation by preconditioning
in [Li, Y., Zikatanov, L.: arXiv:2010.06774 (2020)] and derive new a posteriori
error estimates for Maxwell two-phase interface problems. The proposed error
estimator provides two-sided bounds for the discretization error and is robust
with respect to coefficient variation under mild assumptions. For Maxwell
equations with constant coefficients, the performance of this estimator is
numerically compared with the one analyzed in [Sch\"oberl, J.: Math.~Comp.
\textbf{77}(262), 633-649 (2008)].
