"mindbogglingly huge" is an incredible overstatement. It's not easy to deal with malicious e-mails, but it's also not some unattainable end-goal that only a huge corporation can achieve. A single very motivated person can do it with significant effort, or a small team (3-4) people can do it with average effort (in the setup phase. Maintenance is a 1-2 people affair, and not even fulltime).