SIP question.

I have made a SIP, my SIP work well in most of app except pword.exe!!
The exception is the back key. It work well in other app, but not in
pword.exe.
I invoked IMCallBack::SendVirtualKey(VK_BACK, 0) when my button was pushed,
and IMCallBack::SendVirtalKey(VK_BACK, KEYEVENTF_KEYUP) when user release
the mouse.