(CNN) -- Yahoo CEO Marissa Mayer has apologized for a massive e-mail outage last week, calling the days-long glitch "unacceptable" and a "massive inconvenience."
"This has been a very frustrating week for our users and we are very sorry," Mayer wrote Friday on Yahoo-owned Tumblr as the company was cleaning up the last of a problem that began December 9 for some users.
The problem was a hardware outage in one of Yahoo's storage centers. Mayer said it affected about 1% of Yahoo e-mail's users. While teams began working around the clock on the problem immediately, it was "a particularly rare one" in which different users were affected in different ways.
That caused it to take longer to fix, she said.
Service was restored to more than 99% of affected users by Saturday, although Yahoo's engineers were still working on the problem Sunday.
"The engineering team has been working over the weekend and making steady progress on restoring access to messages for affected users. This includes folders as well as inbox state (like whether you've read a message or starred it)," said a post on Yahoo's help page Sunday night.
Mayer took the helm at Yahoo last year and has been credited with the first steps of what may be a turnaround for the troubled Web giant. She promised improvements to Yahoo's e-mail service, which is the Web's second- or third-most popular, depending upon which analysis you believe.
"Above all else, we're going to be working hard on improvements to prevent issues like this in the future," she wrote. "While our overall uptime is well above 99.9%, even accounting for this incident, we really let you down this week.
"We can, and we will, do better in the future."
Mayer made revamping e-mail a key part of her effort to kick-start Yahoo, but it's been a rocky process.
Announced in December 2012, the first e-mail overhaul rolled out to all users (whether they wanted it or not) in June. So lukewarm was the reception that it got a largely cosmetic redesign again in October. The result? More outrage.
Yahoo e-mail has suffered from other limited outages since October, although none as bad as last week's.