Google has apologised after its Gmail service was knocked offline for 100 minutes, affecting the majority of its 150 million users.
The company said yesterday's widespread outage was a "Big Deal" and it had already carried out a "thorough investigation" into what happened.
The disruption was caused by a lack of capacity which resulted from taking some of Gmail's servers offline for routine upgrades.
Google said it "slightly underestimated" the load that would be transferred to other computers directing Gmail traffic following some other recent changes.
The free version of Gmail is the world's third most popular email program and the disruption is a reminder of the growing dependence on Google's technology.
Google's Ben Treynor, who is in charge of engineering and reliability, wrote in a blog post: "We know how many people rely on Gmail for personal and professional communications, and we take it very seriously when there's a problem with the service.
"Thus, right up front, I'd like to apologise to all of you - today's outage was a Big Deal, and we're treating it as such.
"We've already thoroughly investigated what happened, and we're currently compiling a list of things we intend to fix or improve as a result of the investigation."
He said Gmail engineers were alerted to the problem within seconds, were able to bring the service back online, and had now turned their attention to making sure it did not happen again.
Google's last major outage happened in May when millions of people were cut off from its search engine, email and other online services after too much traffic was routed through computers in Asia.