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Apple Mail was NOT really usable on the Mac at all while was email was downloading from O365. Apple Mail was unresponsive and looked like it was hung most of the time. This is normal behavior. We recommend users leave Apple Mail running to download the emails but do NOT use it to check or send emails while it is busy downloading from the Outlook365 cloud. Instead, use the NEW O365 OWA webmail on your preferred browser which can be found at outlook.office.com . Users who have been migrated to O365 should stop using the old Exchange OWA (owa.mit.edu). It will no longer work for you.
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