Thousands of Yahoo email users have taken to Twitter to complain that the internet company's services have been unavailable for more than six hours.
Users in Spain, France and Germany were the worst-hit, according to outage tracker website Downdetector.
Yahoo said in a series of tweets that it was working to address the issue.
"We understand how difficult this must be for you. We are working especially hard to fix this," Yahoo tweeted on Thursday.
The email outage was trending on Twitter with the hashtag YahooMail. A user tweeted, "Sorry if you have been trying to email me. I'm on yahoo and its borked."
Yahoo later said "most services" were back online, but customers en masse replied to say they weren't.
"How can you say its working??? Its not working," one person said. "Check again and fix it please."
Yahoo, founded in 1995, once ran the most popular email service and was visited by hundreds of millions of users but steadily lost out to Alphabet's Gmail.
Yahoo's services include email, search engine and a news service. The company had rejected a US$45 billion buyout offer from Microsoft in 2008, only to sell its core business for US$4.48 billion to Verizon Communications in 2017.
Verizon combined the business with AOL to form a venture called Oath.
Reuters / Newshub.
