Perimeter Security, a software-as-a-service company that provides managed security to businesses and operates as a wholesaler to service providers, announced today it has merged with USA.NET, a provider of secure e-messaging.

The announcement comes on the heels of Perimeter’s $50 million financing deal with Goldman Sachs, which, along with Bessemer Ventures and the Stripes Group, provided equity funding for the merger.

The arrangement unites two SaaS companies focused on security. Perimeter’s hosted security offering includes firewalls, intrusion protection, gateway anti-virus, desktop anti-virus, anti-spam and Web content filtering while USA.NET offered similar security for e-messaging as well as an integrated suite of secure hosted messaging services, including its Hosted MicrosoftExchange and Commercial Messaging platforms, and mobility options.

Perimeter has been selling more aggressively to service providers in recent years, to enable them to offer a managed security service. The company also sells through value-added resellers and its own sales force. USA.NET brings 1300 new customers to Perimeter’s existing roster of 4000, the two companies said.