Very recently, Microsoft disabled basic authentication for MS 365 Exchange Online, including IMAP and POP3.
https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-and-exchange-online-september-2021-update/ba-p/2772210
This change prevents the ERPNext Inbox feature from working with MS 365, until it’s upgraded to support Microsoft’s new requirements.
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