For what is supposed to be a rival to Google and other major IT companies, Yahoo's inability to get a service as basic as email up and running - after five days - has become a major embarrassment. Late on Friday, Yahoo CEO Marissa Mayer posted an apology to her customers on the company's Tumblr blog, which is working for the time being.

"This has been a very frustrating week for our users and we are very sorry," began Mayer's "Update on Yahoo Mail."  Mayer called the outage, which has been going on since Monday "unacceptable" and "something we're taking very seriously."

After days of silence about the issue, followed by a Twitter message on Thursday telling customers the company was "slowly ramping up... access to ensure stability," and "working tirelessly to restore access to those still having issues," Mayer finally spilled the beans in her post: "Unfortunately, the outage was much more complex than it seemed at first, which is why it's taking us several days to resolve the compounding issues."

According to Mayer, the outage drama began on Monday Dec. 9 at 10:27 pm pacific time, when the network operating center alerted the engineering team for Yahoo Mail that there was a hardware outage in one of the storage service. That single server outage only affected 1 percent of Yahoo Mail users, according to Mayer, so when the engineering team set to work, they estimated that the systems would be fully recovered by 1:30 pm PT on Tuesday. That didn't happen.

Mayer explains: "However, the problem was a particularly rare one, and the resolution for the affected accounts was nuanced since different users were impacted in different ways. Some of the affected users were unable to access their accounts, instead seeing an outdated "scheduled maintenance" page which was a confusing and incorrect message (this has since been corrected and updated). Further, messages sent to those accounts during this time were not delivered, but held in a queue."

Mayer says that the Yahoo team has finally restored access to "almost everyone" and delivered the queued email messages (for those who waited five days, snail mail might have been a quicker option). Now IMAP access is still rolling out, and individual accounts that lost their configurations of starred messages, folder organization, and other individual preferences will be restored by Yahoo. Mayer even promised to communicate with individual affected customers as the restoration continues.

Mayer ended her explanation with the contrite "we really let you down this week." The tone is much different than the confident, playful tone Yahoo's executives took with their own employees in an internal memo in late Nov., after finding out that only 25 percent of people working at Yahoo actually used Yahoo Mail:

"Earlier this year we asked you to move to Yahoo Mail for your corporate email account. 25% of you made the switch (thank you). But even if we used the most generous of grading curves (say, the one from organic chemistry), we have clearly failed in our goal to move our co-workers to Yahoo Mail... Feeling that little tingle? Take a deep breath, you can do this. We want you on board, sailor!"

Yahoo failed in a much more public way this week. Mayer and her team are probably not taking it as lightly this time.