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 .
Yahoo CEO Marissa Mayer admits , ` ` It 's been a very frustrating week '' E - mail has affected 20 % of users , Yahoo says . Mayer has made revamping e - mail a key part of her plan to kick - start Yahoo .