//Source in Hook-DLL
{...}
type
  THookMap = record
    aHandle: integer;
    BlockMouse: boolean;
  end;

var
  HookHandle : HHook;
  FileMapObj : THandle;
  FileMapView : ^THookMap;

function MouseHookProc(Code: Integer; wParam: WPARAM; lParam: LPARAM): LRESULT; stdcall;
var
  BlockM: boolean;
begin
  if code = HC_ACTION then
  begin
    FileMapObj := OpenFileMapping(FILE_MAP_READ, true,'MouseHookMapFile');
    if FileMapObj <> 0 then
    begin
      FileMapView := MapViewOfFile(FileMapObj, FILE_MAP_READ, 0, 0, 0);
      PostMessage(FileMapView^.aHandle, WM_HOOKMAP, wParam, lParam); //hier stimmt noch alles
      BlockM := FileMapView^.BlockMouse;
      UnmapViewOfFile(FileMapView);
      CloseHandle(FileMapObj);
    end;

    {if wParam <> WM_MOUSEMOVE then
      ;}

    if BlockM then //Hier allerdings nicht mehr. Erst wenn die noch auskommentierten Zeilen nicht mehr auskommentiert sind, wird der Wert von BlockM richtig erkannt.
      Result := CallNextHookEx(HookHandle, Code, WParam, LParam);
  end;
end;
{...}

//Source in Programm

{...}
 FileMapObj := CreateFileMapping( INVALID_HANDLE_VALUE, nil, PAGE_READWRITE, 0, SizeOf(THookMap), 'MouseHookMapFile');

  if FileMapObj = 0 then
    raise Exception.Create( 'Error while creating file.');

  FileMapView := MapViewOfFile(FileMapObj, FILE_MAP_WRITE, 0, 0, 0);

  FileMapView^.aHandle := Handle;
  FileMapView^.BlockMouse := false;
{...}

...
 --> {if wParam <> WM_MOUSEMOVE then  <--  ????????????
      ;}

    if BlockM then
      Result := CallNextHookEx(HookHandle, Code, WParam, LParam);
...