Try this.
Private Sub Application_ItemSend (ByVal Item As Object, Cancel As Boolean)
On Error Resume Next
prompts = "Does this need to be sent secure?"
choice = MsgBox (prompts, vbYesNoCancel + vbQuestion + vbMsgBoxSetForeground, "Does this need to be sent secure?")
if choice = vbYes then
strSubject = "[Secure]" & strSubject
elseif choice = vbNo then
strSubject = strSubject
else
exit sub ' User hit cancel, we're done here.
End if
' Send the email here.
if err.number <> 0 then
Msgbox "We creashed. " & err.description
end if
End sub