I recently fielded and answered the same question via email so I apologize if this is repetitive.
The answer is that we do not have a way to link email entries to XpenseTracker entries. We have discovered a clever way of linking attachments found in emails which we are considering incorporating in the app. However, the actual capturing of email text is not something we have at the moment. I would recommend copying the text and pasting it into the XpenseTracker app's entry. If you are running iOS 4, you can quickly jump out of XT, into the email app, do your copy and then easily jump into XT where you left off.