"We haven't heard any reports, any customers calling to say they were affected by this directly," said Deanna Sanford, lead product manager for MSN marketing, Microsoft's Internet division.
"The good news is the Hotmail servers are back up and running now and the issue has been resolved," she said Monday.
About 40 million electronic mail customers were inconvenienced when the site was temporarily shut down Monday night for repairs, reports CBS This Morning News Anchor Julie Chen.
The company said the trouble was fixed by 10 a.m. PDT.
"As soon as we were notified, we got our developers and testers on it, and we were able to determine a fix," Sanford said.
The problem was first reported by the Swedish newspaper Expressen, which said several hackers around the world had set up Web sites offering unauthorized access to Hotmail's accounts simply by typing in a user name.
Users were able to read and send mail from any account without having the password.
The Redmond-based software maker did not know how long the accounts were accessible.
Microsoft posted a note to Hotmail users on its web page Monday, apologizing for the shutdown.
"In light of the inconvenience that such an action can cause users, this is not something that we take lightly but felt that, given Microsoft's commitment to protecting people's private data and information, it was the appropriate course of action," the company said.
"Security and privacy are obviously key concerns," Microsoft spokesman Dan Leach said Monday night in a telephone interview.
"Everything was brought back on line and everything is fine," he said.
Leach said he knew of no other Microsoft products or services that might be similarly vulnerable.