The call to NotesDocument.GetMIMEEntity will return Nothing if the Person document field labeled "Format preference for incoming mail" (MessageStorage item) is set to "Prefers Notes Rich Text" (0). The installation program should check for that item value and display an error message. And then the agents that check for spam should also try to see if the $NoteHasNativeMIME item is set to "1"; if not then there is no way to check for spam so log an error.