No. All shadow receive checkboxes are activated, including simobjects.
If with "all", you really mean "all", this might be a problem because, if you also checked the "No shadow flagged content", it means you are allowing even the objects we explicitly flagged to not cast shadows, to cast them anyway so, even the background it's casting a shadow (it shouldn't) so, consider you also seems to have two backgrounds in conflict, one is casting a shadow over the other one.