Katie Collins/CNET

Microsoft inadvertently opened the spam floodgates for users of its Outlook.com email service for nearly 17 hours before fixing the problem Wednesday.

These days we rely on email services to shield us from countless messages stuffed with financial scams and pharmaceutical product offers. But it was tough to sift the wheat from the chaff on Outlook.com for a good chunk of the workday Tuesday.

"Some users may have been receiving excessive spam mail," Microsoft said dryly of the problem on its service portal. It was fixed at 4:52 a.m. PT Wednesday.

Google's Gmail, the top-ranked email service, considers spam blocking a major feature. Using artificial intelligence technology, the company claims Gmail can screen out all but 0.1 percent of spam.