While users of the standalone Gmail service for individuals and of the Standard and Education editions of Apps don't pay for it, Gmail is part of the fee-based Premier version of Apps, which costs US$50 per user per year. Apps Premier includes a 99.9 percent uptime guarantee for Gmail and for other suite components.

Like other vendors of Web-hosted applications, Google maintains that the SaaS model offers significant advantages over conventional software that users and organizations have to install on their PCs and servers, also known as "on-premise" software.

Those advantages include not having to spend time and resources maintaining and updating the software; a lower overall cost of ownership; and the ability for workgroups to better collaborate on documents because files reside on a server, where multiple people can access and edit them.

However, concerns remain among many CIOs and IT managers over storing corporate data on software vendors' data centers and over lack of control during outages such as the one on Tuesday.

Although in recent years Gmail has had several well-publicized and widespread outages, as well as other ones smaller in scale, Google maintains that the service is inherently more stable than on-premise installations of competing messaging software like Microsoft Exchange and IBM's Lotus Notes.