Online giant Google Inc. has signed a definitive agreement to acquire communications security and compliance firm Postini . Postini was acquired for $625 million in cash.

Postini services, which include Email Security, IM Security, Web Security, Message Archiving, Message Encryption and Policy-enforced TLS, will now be available on Google Apps, which is compromised of Gmail, Calendar, Talk, Docs & Spreadsheets, and Personal Start Page.

Google will gain access to Postini’s 35,000 clients and Postini to Google’s 100,000 clients. There is very little crossover between the two audiences, according to Mr. Girouard. Even though cross-platform integration will be offered, clients can contract each firm individually. Google will continue to support Postini customers and invest in Postini products.

Experts are seeing this merger as another big step which will create the next generation in on-demand service.