Some research has shown that as much as three quarters of a company's business critical data can be in the form of email archives. The problem is aggravated by the ability to create pst files, which often can circumvent server size limits. this encourages employees to save their data as past files. We know that the use of email has exploded in recent years. Also it has been widely known that more and more critical and sensitive data is often included in emails. So it is no surprise that research is beginning to show the extent to which data is stored in email archives. The implications for systems are clear - we need to formalize approaches to the use, maintenance and security over email systems and make sure they are subject to the same high levels of integrity as other aspects of the business systems.
http://www.computerworld.com/pdfs/messagelabs_death_to_pst_pdf.pdf