Microsoft has resolved this issue. We apologize for any inconvenience this may have caused, and appreciate your patience. If you have any questions or continue to experience issues, please contact your support team.
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Microsoft has advised that a recent service maintenance change introduced a code issue that inhibits communication between legacy Outlook for Mac users and the Exchange Online service, which is resulting in the impact. They are validating a fix to resolve the issue which they hope to implement later today. As a temporary solution, we recommend using webmail for accessing your 365 email or opting for "New Outlook" in Outlook for Mac.
We apologize for any inconvenience this may cause and thank you for your patience. If you have any questions, please contact your support team.