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Hi Charlie Cho
Thank you for reaching out to Microsoft Q&A forum.
First, I hope you understand that this forum is a peer-to-peer support community. I am also a fellow user, and the guidance I provide is based on my own research and Microsoft’s published documentation.
From what you’ve described, and after reviewing similar cases, this appears to be an intermittent issue. Other users have reported occasional problems with Mac Mail and Exchange, and in most cases the issue resolved itself after a few hours or a few days.
For your reference, here are some related scenarios you can review. While the error symptoms are not identical, they all share the characteristic of being intermittent issues.
Unable to add Exchange email address in Apple Mail on Mac - Microsoft Q&A
Unable to add my hotmail addresses to imac - Microsoft Q&A
Outlook email Quit working on my Mac suddenly - Microsoft Q&A
Apple Mac Mail无法添加我的Hotmail和Outlook账户 - Microsoft Q&A
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I hope this information helps.
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