If you are encountering an issue with Outlook on your Mac after transitioning to Microsoft 365 it may need the server address changed in settings. Please follow the instructions below to change to make the necessary changes.
Open Outlook on your Mac
Select Outlook > Preferences > Accounts, select the Exchange email account from the list, and then click Advanced.
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Change the address listed in the Microsoft Exchange Server box to https://outlook.office365.com/EWS/Exchange.asmx
If something is listed in the "Directory Services" area, please remove any of those settings.
Click OK
Close Outlook and then re-open to view the updated mail.