An outage at Yahoo Mail stretched into its second day on Wednesday, prompting the US Internet pioneer to apologize to unhappy users.
Yahoo would not disclose the number of email accounts knocked out of service since late Monday night California time.
"We are very sorry for recent difficulties with Yahoo Mail," the company's senior vice president of communication products Jeffrey Bonforte said in a short message posted at Tumblr.
He blamed the outage on a "hardware problem" at one of Yahoo's data centers.
"The issue has been harder to fix than we originally expected," Bonforte said. "We have dozens of people working around the clock to bring it to a resolution."
Explore further: Yahoo buys image-recognition startup for Flickr