Cisco Systems is making its largest security acquisition ever, as it pushes well beyond the network into the application layer--and toward subscription pricing.

Cisco plans to acquire IronPort Systems, a provider of e-mail and Web security and security management appliances, for about $830 million in cash and stock. It would be Cisco's fifth-largest acquisition overall.

IronPort itself is buying PostX to help companies better control the information coming and going via e-mail. Using IronPort's Web- based Email Security Manager with PostX, which encrypts e-mail, message recipients can access e-mail only after authenticating through a Web interface, and senders can recall messages sent erroneously. IronPort also has a deal to use Vontu's software for identifying sensitive data. Combined with IronPort's anti-spam and antivirus software, it's meant to check for sensitive data in outbound e-mails or files.

IronPort's subscription-based pricing model for the filters and other software on its appliances appeals to Cisco, though it hasn't decided which products fit the model. "Many areas would lend themselves to be delivered in a software-as-a-service or software subscription model," says Jeff Platon, VP of Cisco Security Solutions marketing.

Cisco's backing also gives IronPort deep pockets to support its messaging security technology--a factor not to be underestimated given how difficult it is for vendors to keep up with the pace of emerging security threats.