In The Official Gmail Blog, Site Reliability Manager Acacio Cruz followed up his earlier acknowledgment of the incident with a posting Tuesday that outlined the cause. "Lots of folks are asking what happened," he said, adding that the explanation involved routine maintenance in one of the European data centers.

'Unexpected Side Effects'

Typically, he said, this kind of maintenance "causes no disruption because accounts are simply served out of another data center."

But in this case, he said there were "unexpected side effects of some new code that tries to keep data geographically close to its owner," and that caused another data center in Europe to become overloaded. From there, the problem cascaded from one data center to another, and he reported that it took Google engineers "about an hour" to get it back under control.

At this point, Cruz wrote, the bugs "have been fixed and found," and the company is in the process of deploying software changes so this kind of event can be avoided in the future. He added that Google's personnel know firsthand "how painful an outage like this is," since the company itself runs on Gmail and outages affect them as well as everyone else.

According to news and blog reports, the outage occurred between 1 a.m. and 7:30 a.m. EST Tuesday morning. The BCC reported that service returned for many users after about four hours, which would still make it one of the longest outages for Gmail.

With more than 110 million users worldwide, Gmail is the third most popular e-mail service, after Hotmail and Yahoo. It's also part of Google Apps, a suite of online business applications used by more than a million businesses worldwide.

Google Apps' service-level agreement for businesses provides that Gmail and the other services will be 99.9 percent operational each month. If the uptime is lower, paying customers could be eligible for a refund. According to some news reports, Google will offer 15 days of free service to paying businesses.

Google Talk Phishing Attack

Google's momentary weakness was an opportunity for scammers. There were reports of a phishing attack that targeted the Google Talk chat service. When users clicked a link to "check out" a video, they were sent to a site called ViddyHo and asked for their Gmail usernames and passwords. Users who gave their confidential information then had a similar message sent to their online contacts, according to news reports. Google has now blacklisted the site and blocked the link.

This brouhaha around Gmail could have an impact on Google Apps' acceptance in small and midsize businesses, according to IDC analyst Merle Sandler. "If your e-mail is your lifeblood," she said, "and you thought someone was trying to steal your password," it could make businesses think twice about beginning to use Google's applications.

But, she added, it's unlikely at this point that existing Gmail and Google Apps users would leave, because it would require substantial effort -- like changing the e-mail address on your business card. For those people, Sandler said, "it is more likely to simply be annoying."