Some of Microsoft's biggest customers haven't had access to email for 9 days

The email component of Office 365, the online version of Microsoft's Office suite, has been down for nine days, according to CloudPro.

Microsoft told CloudPro that the issue was not affecting all Office 365 users, but "those customers affected likely have a large number of users experiencing impact."

It's unclear why the outage, which affects one of Microsoft's most important products, has been going on for nine days straight.

According to CloudPro, the issues started on January 18. "As part of our efforts to improve service performance, an update was deployed... [that] caused a code issue," said Microsoft.

The fix was temporary, however, as users reported outages on January 24. Microsoft said it had fixed that issue, too, but companies are still not able to access email.

"Engineers have identified that a number of infrastructure components that handle email routing and filtering became degraded due to high resource utilisation," said Microsoft in a statement issued on January 26. "Engineers are restarting services on the affected infrastructure to mitigate impact."

One company, highlighted by CloudPro, says that OneNote Online and Office 365 are still refusing to open, nine days later.

Microsoft has experienced four outages in the past two months.

Business Insider has reached out to Microsoft to ask about the outage. We will update the post when we hear back.