Well, that sounds pretty straightforward actually. You need to index your mail based on various criteria, and then create a way to view them that links to other things related through those criteria. The obvious approach would be to use a database, build an indexer program that you can feed your mail to, and build a web application for displaying and cross-linking. The first step would be to decide which things you will cross-link on and work on creating a database schema and figuring out how to extract them from an e-mail.