Service had been restored on the Web shortly after 2 p.m. PDT Wednesday, according to an update on Microsoft's website. That's more than seven hours after Microsoft first acknowledged that some people had been locked out of their Outlook.com accounts.
Some Outlook.com users trying to check email on mobile devices might still be encountering trouble that engineers were trying to clear up, according to a Microsoft statement.
Earlier in the day, some people also could not retrieve their files stored on Microsoft's SkyDrive service. But that problem had been resolved by late morning PDT.
Microsoft blamed the breakdown on an unexplained "incident." The Redmond, Wash., company said only a small number of users were affected, but didn't provide further specifics.
Even if only a small percentage of Outlook.com users were cut off, millions of people could have been inconvenienced. Microsoft boasts it has more than 400 million accounts on Outlook.com.
Microsoft Corp. transferred all the users of its other Web email, including those with the hotmail.com and msn.com domains, to Outlook earlier this year as part of its attempt to outshine Google Inc.'s Gmail and Yahoo Inc.'s email.