Two days ago, Yahoo CEO Marissa Mayer tweeted that its ongoing problems with Yahoo Mail had finally been fixed. But she spoke too soon: email problems continued, drawing mounting criticism for the company's inability to restore the service. The outage comes at a time when the company is still struggling to persuade its own employees to use Yahoo Mail instead of Outlook for corporate email. Today, Mayer took to Tumblr to try to explain what had happened. "This has been a very frustrating week for our users and we are very sorry," she wrote.

"This has been a very frustrating week for our users."

According to Mayer, the outage was "much more complex than it seemed at first." The problem began Monday night, when Yahoo Mail suffered a hardware outage in one of its storage systems. But different users reported different problems, making the outage more difficult to resolve. Some people were locked out of their accounts; others failed to receive messages. (Mayer says they were held in a queue.) The outage affected about 1 percent of Yahoo Mail users, she said.

Mayer said that Yahoo has restored Mail access "to almost everyone" and delivered their messages to them. Affected users will be notified by Yahoo directly on the status of their account, she said. "Above all else, we're going to be working hard on improvements to prevent issues like this in the future," Mayer said. "While our overall uptime is well above 99.9%, even accounting for this incident, we really let you down this week." Future updates will be posted to Yahoo Mail's status page.