In MS Outlook, report all sent-messages that haven't received replies

As Jez suggested, look at the message id of the sent message (you can access it using MailItem.PropertyAccessor), you can then try to find the matching message in your inbox folder by reading the PR_TRANSPORT_MESSAGE_HEADERS property (again, use MailItem.PropertyAccessor, the property will contain the MIME headers from the main MIME part of the message) and extracting the "In-Reply-To" and "References" headers.
You can see the relevant properties along with their property tags and DASL names in OutlookSpy (click IMessage button).