  We study the plasma-vacuum interface problem in relativistic
magnetohydrodynamics for the case when the plasma density does not go to zero
continuously, but jumps. In the vacuum region we consider the Maxwell equations
for electric and magnetic fields. We show that a sufficiently large vacuum
electric field can make the planar interface violently unstable. By using a
suitable secondary symmetrization of the vacuum Maxwell equations, we find a
sufficient condition that precludes violent instabilities. Under this condition
we derive an energy a priori estimate in the anisotropic weighted Sobolev space
$H^1_*$ for the variable coefficients linearized problem for nonplanar
plasma-vacuum interfaces.
