Yahoo chief Marissa Mayer posted an apology late Friday for a "frustrating week" experienced by millions of its email service users.
"This has been a very frustrating week for our users and we are very sorry," Mayer said in a message at Yahoo-owned blog platform Tumblr.
"For many of us, Yahoo Mail is a lifeline to our friends, family members and customers," she continued.
"This week, we experienced a major outage that not only interrupted that connection, but caused many of you a massive inconvenience."
A hardware problem in a data center late Monday knocked out a storage system serving about one percent of Yahoo Mail users, according to Mayer.
Yahoo Mail is reported to have more than 280 million users, putting the number of people affected at nearly three million.
Furious users vented rage and frustration on Twitter, Facebook and other social media venues.
Many people said they couldn't retrieve emails from their inboxes or complained that emails were not being delivered.
Ranks of Yahoo Mail users vowed to switch to rival services such as Gmail, but it remained to be seen whether they will follow-up with action.
"The problem was a particularly rare one, and the resolution for the affected accounts was nuanced since different users were impacted in different ways," Mayer said.
Some people trying to access their Yahoo Mail accounts got incorrect messages that "scheduled maintenance" was being done. Email sent to Yahoo accounts was not delivered.
Mayer said that as of late Friday, access was restored to nearly all Yahoo Mail users and backlogged messages were delivered.
Yahoo was still working on getting the free Web-based email service completely back in order.
The Yahoo Mail disruption marks a stumble for Mayer, who has made focusing on people's "daily habits" such as email part of her plan to revive the faded Internet pioneer.
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