SendMessage WM_NOTIFY across process

Hello,

I'm using a program to drive a GUI. For simple messages such as
LB_SETSEL the PostMessage API works fine. I use SendMessage for
WM_NOTIFY as it takes the address of a local NMITEMACTIVATE structure.
I've tried this with very simple cases. Using Spy++ to obtain the
window handles and to observe the messages I never see the WM_NOTIFY
message neither does my simple handler in my simple client get
invoked. I'm assuming window handles are globally unique. Here is my
code on the testing side:

void CTelusDlg::ClickOnListControl(HWND hWndParent, HWND hWndListCtrl,
int iItem, int iSubItem)
{
   NMITEMACTIVATE nmia;
   ::ZeroMemory(&nmia, sizeof(NMITEMACTIVATE));
   // Set up header
   nmia.hdr.code = NM_CLICK;
   nmia.hdr.hwndFrom = hWndListCtrl;
   nmia.hdr.idFrom = ::GetDlgCtrlID(hWndListCtrl); // value is 426
   // Setup item and subitem
   nmia.iItem = iItem;
   nmia.iSubItem = iSubItem;
   if ( ! ::SendMessage(hWndParent, WM_NOTIFY, nmia.hdr.idFrom,
(LPARAM)&nmia) )
   {
      DWORD dwError = ::GetLastError();
      LPTSTR lpBuffer;
      ::FormatMessage( FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM,
                           NULL,
                        dwError,
                           MAKELANGID(LANG_NEUTRAL, SUBLANG_SYS_DEFAULT),
                           (LPTSTR)&lpBuffer,
                        0,
                        NULL);
      ::AfxMessageBox(lpBuffer);
      ::LocalFree(lpBuffer);
   }
}

Here's the other side:

BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
    ON_WM_CREATE()
   ON_NOTIFY(NM_CLICK, 426, OnClick)
END_MESSAGE_MAP()

void CMainFrame::OnClick(NMHDR *pNMHDR, LRESULT* pResult)
{
   ::AfxMessageBox(_T("Hurrah!!"));
   ASSERT(NM_CLICK == pNMHDR->code);

   NMITEMACTIVATE *pNMIA = (NMITEMACTIVATE *)pNMHDR;

   *pResult = 1;
}

I'm beginning to think that something chokes WM_NOTIFY messages when
they're sent to windows in different processes. Am I right or has
somebody managed this? Is there a workaround?