Google's Gmail service was down for most of its users worldwide for almost two hours, affecting not only individuals users but also organisations who depend on it as their business email system.

Google acknowledged that Gmail had a widespread, worldwide collapse shortly before 4.00 in the afternoon . US Eastern Time (9 PM BST) and declared the outage resolved at close to 5:40PM., according to information posted on the Google Apps Status Dashboard.

In that dashboard, Google offers performance and availability information about the online services included in its Apps collaboration and communication suite, such as Gmail, Calendar, Talk, Docs and Sites.

During the outage, "users could continue to send and receive e-mail using POP and IMAP," said a Google spokesmanl.

Google is investigating the cause of the outage and hopes to share its findings soon, Google engineering director David Besbris said in an official blog posting.

The outage followed another of a smaller scale at mid-afternoon Monday that apparently wasn't fully solved until late Tuesday morning.

Gmail is one of the most popular webmail services in the world, not only among individual users but also as a component of Apps, which is designed for use by organizations of all sizes and educational institutions.

As such, Gmail outages are immediately felt worldwide, especially among people and companies that use it for business communications.

Google has said in the past that it provides the Gmail service to all types of users using the same infrastructure, which is why outages, no matter their scope, typically affect all types of users indiscriminately.

Gartner analyst Matt Cain believes this is a mistake on Google's part. "The critical issue for Google is to segment commercial Gmail traffic from consumer traffic," he said.

"Google must prove that consumer and commercial services are largely independent of each other, and that the commercial services will sustain at least a 99.9 percent uptime, which is the standard for most commercial e-mail SaaS [software-as-a-service] services," Cain said.

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 yesterday.

Although in recent years Gmail has had several well-publicised 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.