E-mail traverses the public Internet in plain text, and can be easily copied or intercepted. Just ask the Gmail users famously hacked in 2010 and again in 2011, or the U.S. Chamber of Commerce which had at least six weeks worth of e-mail data containing sensitive information stolen in a breach that was widely reported in December 2011.
Today, as increasing global competition drives the need for increasing inter-enterprise collaboration, the use of e-mail to share collaborative information with partners and geographically dispersed team members creates unacceptable security, compliance, and information governance risks. If your users are collaborating on a sensitive project and sharing a confidential document or data, the last thing you want them to do is e-mail it back and forth over the Internet.
I provide some strategies to get around this issue in a white paper that I wrote for Intranlinks here.