Microsoft are currently investigating an issue that is currently preventing some of its users from accessing Exchange, the issue appears to be primarily affecting users who are unable to access their mailboxes.
A networking configuration is believed to be what Microsoft is the cost of the issue and are currently investigating. At this time there is not a known timeline for restoration.

Microsoft have posted via Twitter that they are aware of the issue and have alerted O365 admin accounts within the O365 Portal.

Down Detector, a service that tracks the status of websites and online services has posted a heat map of where this is being impacted most.