Haw many staff *really* need to send external email?
So, you're sitting at your desk in the chicken-farm. Along with all the other drones: in long lines, each with the company approved PC, phone, chair and pen. What part of your job requires you to send emails to your personal email address / your friends / other companies (who aren't on the approved supplier list)? You may even ask: what part of your job involves surfing, using facebook, linkedin (I didn't know that was still going), twatter etc. - not that you could send much confidential info in each twot, but I digress.
Maybe, rather than employ staff to oversee, censor and inform on all these dubious activities, why not save some headcount, increase productivity and cut virus infections AT A STROKE by axeing internet access. Let's face it, most orders are send without any human intervention, most emails get ignored or misinterpreted and the only real way to get high quality information is face-to-face: with phone calls a very poor second - maybe even third choice, after posting pieces of paper.
Not that this would have any effect on leaks of real information. That would still get left on trains, smuggled out on thumb-drives, stolen on unsecured laptops and slipped off the MD's tongue after the 4th G&T at the golf club.
All you'd really do (apart from massively increasing your staff productivity) would be forcing them to invent other excuses about why the business is doing badly. Most organisations don't actually have any secrets worth a damn, anyway.