According to DownDetector, an online tracker of service outages, the number of users reporting issues has steadily increased since about 5 a.m. ET Thursday morning.

Microsoft 's Outlook email service malfunctioned for several hours Wednesday and Thursday, prompting some people to post on social media about the inability to reach their virtual mailboxes.

"Users may be unable to access their mailbox using any connection methods," the page read, noting that the company was working to fix the issue.