Yahoo believes the problem will be fixed by 3 pm PST on Wednesday, according to a post on Yahoo's official Tumblr blog by the company's senior vice-president of communications, Jeffrey Bonforte.

Bonforte did not specify how many users were affected by the email outage, which he said began at 10:27 pm PST on Monday. He said the company has "dozens of people working around the clock" to resolve the problem.

"The issue has been harder to fix than we originally expected," Bonforte said.

Yahoo Mail is the most popular Web-based email product among desktop computer users in the United States, according to analytics firm comScore. Yahoo Mail ranks second worldwide, behind Google Inc's Gmail and ahead of Microsoft Corp's Outlook.com.

A redesigned version of Yahoo Mail, which the company introduced in October, has met with a mixed reaction by users. Many users have griped that the new version eliminated popular features such as "tabs."

(Agencies)