How to read only new content in VSTO Outlook MailItem body?

Question

I've written a little C# VSTO add-in for Outlook 2003 that reads the body of emails as they are being sent, looking for certain words. It's working right now to do this:

if (currentItem.Body.Contains("text to search for"))

... but that checks the entire email body, not just the new message being sent.

Is there anyway to have Outlook just check the contents of the new message being sent, and so ignore the older email chain that might be in there too?

These messages could be in any format (HTML, Rich Text, Plain Text) and may or may not have any earlier messages chained in. This is just a productivity tool for me, so any hack that works is worth considering here.

Thanks!