If your business runs Exchange and you are not running some type of system that archives your e-mail then there is a good chance you are breaking the law. The Sarbanes Oxley act of 2002 specifically requires some form of e-mail retention as e-mail is often the deciding factor in various types of corporate litigation.
If you are a systems administrator, this means you need some type of third party software or hardware that will perform this task. A couple of the more popular options include hardware solutions such as the Barracuda Mail Archiver, as well as software solutions such as GFI MailArchiver or Symantec Enterprise Vault. Also, if your organization runs any type of enterprise level document management software, you may already have access to a feature or add-on such as this. Different products work better for different types of networks, so it is best that you evaluate these and many of the other e-mail retention products thoroughly.