A recent article in the New York Times highlighted the risks associated with your employees using unmonitored WebMail services.

"A growing number of Internet-literate workers are forwarding their office e-mail to free Web-accessible personal accounts offered by Google, Yahoo and other companies. Their employers, who envision corporate secrets leaking through the back door of otherwise well-protected computer networks, are not pleased."
                                                Firms fret as office e-mail jumps security walls
                                                New York Times, January 11, 2007