Microsoft today said it restored full service to Hotmail and Outlook.com around 6 a.m. PT, about 15 hours after the online mail sites went partially dark.
The problems, which surfaced Tuesday afternoon, affected not only Hotmail and Outlook.com -- the latter a rebranded, revamped version of the former -- but also Microsoft's cloud-based SkyDrive storage service.
SkyDrive was back up and running later Tuesday, but Hotmail and Outlook.com continued to block many customers from accessing their accounts through the night.
"Fixing the problem is taking longer than we hoped," Microsoft reported several times in the early hours of Wednesday on its service status page. "We apologize for the lengthy interruption in service."
Hotmail and Outlook.com were offline for some users until 5:43 a.m. PT -- just over 15 hours after the outage's start -- when Microsoft gave the all clear.
"Hotmail and Outlook.com experienced an outage that affected customers' access to their email," Microsoft said in a statement. "Fortunately, the issue has been resolved and service is now restored to normal. We sincerely apologize for any inconvenience this caused our customers."
The outage was an embarrassment for Microsoft, which moved Outlook.com out of beta just three weeks ago, and has been touting the service in a new "Scroogled" campaign of attack ads aimed at denting Google Gmail's reputation.
Some Office 365 users also reported that their Office-by-subscription service had been disrupted.
Microsoft did not immediately reply to questions about whether Office 365 had been impacted along with Hotmail, Outlook.com and SkyDrive. Some Office 365 plans rely on SkyDrive for document storage.
Along with the outage, Microsoft also conducted maintenance on its Calendar yesterday for about nine hours, then repeated that today. "Hotmail is performing maintenance on the Calendar until 6 p.m. PDT," the company's status page stated, adding that viewing and editing could be slow, that error messages may appear, and that synchronizing the calendar might be delayed.
The Hotmail, Outlook.com and SkyDrive services have logged numerous interruptions so far this year, including March 5; Feb. 1 and Feb. 7; and Jan. 1, Jan. 3-4, Jan. 7, Jan. 8, Jan. 13 and Jan. 31.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is firstname.lastname@example.org.