Yahoo! Inc., the Web portal that revamped its e-mail service in October, apologized for a problem that has knocked out access to the service for some users since Dec. 9.

The company had “dozens of people working around the clock to bring it to a resolution,” Jeffrey Bonforte, a senior vice president at Yahoo, said on the company’s website. “The issue has been harder to fix than we originally expected.”

Earlier today, Yahoo said it expected access to be restored by 3 p.m. San Francisco time. As of 5 p.m., most users should be able to log in and send and receive messages, Yahoo said in an updated posting. Some recent messages may not appear in users’ inboxes immediately as the backlog of mail from the outage is cleared, the company said.

“We’re still working on bringing all accounts up to date,” the company said on its help website.

The outage adds to the troubles that have beset the e-mail platform -- used by more than 100 million people daily -- since Chief Executive Officer Marissa Mayer redesigned the service. Users have complained about software issues as well as the look and organization of the new mail site, including searches for older messages and new characteristics, like conversation threads, that mirror Google Inc.’s rival Gmail service.

“I get a ‘scheduled maintenance’ message saying ‘some users may experience problems accessing Yahoo Mail,’” said Ellen Beth Levitt, a Yahoo Mail user in Baltimore, who said she couldn’t reach anyone in customer service via phone or e-mail. “It’s the e-mail I use for everything personal and I’m afraid I’m missing important communications.”

The shares of Yahoo, based in Sunnyvale, California, fell 2.7 percent to $39.16 at the close in New York. The stock has increased 97 percent this year. (Bloomberg)