When users are unable to access their mailbox or SharePoint on Office 365, this can sometimes be due to sync issues. Most organizations will use AAD connect (previously called DirSync, and AD sync) to sync their on-premise identities to their Office 365 environment, so it’s important that these two systems communicate to make it possible for users to access what they need on Office 365.
For a five-hour period on December 3, 2015, European Office 365 tenants discovered that some of their users were unable to access their mailboxes or SharePoint sites. Interestingly, the problem was not caused by an issue with the Office 365 infrastructure, but with a configuration error in Azure Active Directory (Azure AD) that affected critical authentication processes. Based on Microsoft’s post-incident report, it seems that this configuration snafu was due to human error in the management of Azure AD, which goes to show how critical Azure AD is in keeping Office 365 services up and running.