It is not the first time Gmail has hit difficulties
Google has issued an apology after a "miscalculation" caused a blackout of its Gmail service, affecting the "majority" of its 150 million users.
The firm described Tuesday's two-hour outage as a "big deal" and said it was investigating ways to ensure it did not happen again.
The disruption was caused by an error during "routine upgrades" to the company's web servers, it said.
It follows outages of the Gmail service in February and March.
"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," said the firm's Ben Treynor in a blog post.
"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."
He said the problem occurred when engineers took some of Gmail's servers offline to perform routine upgrades.
"This isn't in itself a problem - we do this all the time, and Gmail's web interface runs in many locations and just sends traffic to other locations when one is offline," he said.
However, said Mr Treynor, engineers "slightly underestimated" the increased load put on other parts of the system during the upgrades, causing the "widespread outage" of its webmail.
Other ways of accessing the service - such as through desktop email programs - were unaffected, the firm said.
Google's last major technical problem happened in May, with millions of people unable to use its main search page, as well as Gmail and Google News.
The free version of Gmail has been ranked as the world's third most-popular e-mail program, behind similar services provided by Microsoft and Yahoo.