SAN FRANCISCO — Even for Yahoo Inc., a company with a lengthy track record of failure, this was an epic fail.

For five days this week an untold number of its 100 million users couldn't send or receive email, prompting many longtime, loyal users to savage the company on social media and threaten to leave for another email service.

Despite the misery the outage caused people and businesses — who could have sent a letter faster than they could an email — Yahoo was slow to respond and even slower to inform users about it.

And that has cast a shadow over Chief Executive Marissa Mayer's yearlong effort to turn around the troubled Internet company.

Peter Sealey, adjunct professor of marketing at Claremont Graduate University, said the handling of the email outage was a major public relations stumble and a "blemish" on Mayer's tenure as CEO.

Colin Gillis, an analyst with BGC Partners, called the outage "an embarrassment."

For all of the Sunnyvale, Calif., company's challenges over the years, some 800 million people log into Yahoo each month to check their email, get stock quotes and scan headlines.

Yahoo Mail is one of the main ways the company keeps a hold on users. Mayer identified Yahoo Mail — one of the world's largest email services — as one of the company's biggest missed opportunities that she planned to fix.

But so far, Yahoo email users have not been happy with the results. A redesign has triggered loud protests and many users say the service is no longer reliable.

Even Yahoo employees don't want to use it. In an internal memo leaked to All Things D tech blogger Kara Swisher, Yahoo management begged employees to switch their corporate email accounts from Microsoft Outlook to Yahoo Mail.

According to Swisher, the executive in charge of Yahoo Mail, Jeff Bonforte, joked at a staff meeting last month that while users might be dissatisfied with changes to the product, the company would have to give them a swift kick in a delicate body part to get them to stop using it.

Yahoo is now issuing frequent updates on its progress in fixing the outage. Still, many users may be missing a couple of weeks of email, and some users were still reporting problems Friday.

Yahoo did not respond to questions about the outage, including how many people were affected and how many may have dropped the service.

Dwana De La Cerna, a government worker who lives in Chicago, says she has been frustrated because her 17-year-old son is waiting to hear from his No. 1 college choice, Indiana University. He has only one email account, Yahoo, and that's the one he gave the school, she said.

"We kept wondering why we hadn't heard anything," she said.

Kathy Badertscher, director of content with public relations agency Knock Twice, says she's had a Yahoo Mail account since 2000. It was her first personal email address. She stuck with Yahoo after opening a Google Gmail account in 2007.

Even before the latest prolonged outage, Badertscher said email access had become "more spotty." And she was not a fan of the redesign. She says she has slowly begun to use Gmail more, and now she's thinking about switching over permanently to Gmail.

"Frustrated? Yep. Disgusted? A little," she said.

Alienating loyal users such as Badertscher won't help Mayer in her difficult quest to breathe new life into Yahoo products, jump-start its advertising business and transform the company culture.

The former Google Inc. executive joined Yahoo in July 2012 to help revive a company that had not kept up with the rapid migration to mobile devices.