Proofpoint touts its new SaaS spam filter as an add-on to an e-mail security appliance. Actually, it's a teaser to get customers hooked on the cloud.This week Proofpoint launched a SaaS-based anti-spam service called Shield. The service is purposely designed to reduce the load on premises e-mail security appliances by pre-filtering large volumes of spam. The service can be deployed in front of Proofpoint's appliances or competing appliances from Cisco, Symantec, etc.

The stated goal of the service is to help companies cope with traffic spikes, such as a high volume of spam or a directory harvest attack. The service rejects known spam and can throttle mail traffic. These actions reduce the loads on premises e-mail security products. Proofpoint says the service also means companies don't have to overprovision premises hardware to deal with mail spikes -- in other words, don't buy anymore appliances.

That's a legitimate use, but it raises a question: If you're already filtering in the cloud, why bother with the appliance at the premises?

My guess is that Proofpoint wants customers to ask the very same question.

I'd say the e-mail security appliance market is pretty well saturated by now. There are very few customers who don't have such a product, so most of the market activity is getting an existing customer to buy a bigger model with more features, or replacing a competitor. This can be a decent business profit-wise, but it doesn't have the growth curve that makes a company's fortune.

Cloud-based mail services, however, are wide open. Shield is a gateway drug. Give the customer a little taste of what the cloud can do. Once they're comfortable with it, they start looking at the bills and ask, "Why am I paying to maintain this appliance when 90% of my filtering is already done off-premises? Let's tweak those filters in the cloud and let the maintenance on this box lapse."

It's same thing as cell phones vs. landlines: As cell phone adoption curves up, landline subscriptions trend down.

Of course, there will always be a set of customers that don't want anything to do with the cloud, so Proofpoint has them covered, too.

I think other vendors see the writing on the wall. Last year Symantec acquired MessageLabs, which provides antivirus, anti-spam, and archiving services.