  For the low density filamentary and sheet-like structures in the warm ($\sim
10^4$ to $\sim 10^6$ K) IGM predicted by numerical simulations the resonant
line scattering of X-ray background (XRB) photons by He and H-like ions of
heavy elements can exceed the ``local'' thermal emission by a factor of a few
or more. Due to the conservative nature of scattering this resonantly scattered
radiation can only be identified if a significant fraction of the XRB is
resolved and removed. While the combined spectrum of the resolved sources will
contain X-ray absorption features, the residual background will contain
corresponding emission features with the same intensity. At the relevant
densities and temperatures the lines of He and H-like oxygen at 0.57 and 0.65
keV are most promising. These lines (which have a typical width of $\sim$ 1-2
eV) may contain up to 50% of the total 0.5-1 keV emission of the filament. For
a nearby ($z \la 0.1$) filament with a Thomson optical depth of $10^{-4}$ XMM
should detect about 200 photons in the OVII line during a $10^5$ s exposure if
the metallicity of the gas is as large as observed in galaxy clusters. On
average up to a few percent of the soft XRB could be resonantly scattered by
this phase of the IGM and resonantly scattered photons should account for a
significant fraction of the truly diffuse background at low energies. Close to
bright X-ray sources like galaxy clusters or AGN the flux of scattered
radiation will be further enhanced. Off-line blazars are the most promising
illuminating sources. The scattered emission from AGN may also constrain the
duration of the active phase of these objects.
