Microsoft disabled SMTP auth by default some time ago. Please make sure it's actually enabled at https://admin.exchange.microsoft.com/#/settings by unchecking [the box shown in this screenshot](https://i.imgur.com/xPf9wG1.png) at Settings -> Mail Flow.
Bug 1775077 Comment 10 Edit History
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Microsoft disabled SMTP auth by default some time ago. Please make sure it's actually enabled at https://admin.exchange.microsoft.com/#/settings by unchecking [the box shown in this screenshot](https://i.imgur.com/xPf9wG1.png) at Settings -> Mail Flow. Note: It may take a few minutes for the SMTP server to reflect the change. It did for me.
Microsoft disabled SMTP auth by default some time ago. Please make sure it's actually enabled at https://admin.exchange.microsoft.com/#/settings by unchecking [the box shown in this screenshot](https://i.imgur.com/xPf9wG1.png) at Settings -> Mail Flow. Note: It may take a few minutes for the SMTP server to reflect the change. It did for me. If you have this box unchecked already, and still can't send, an error from the error console in Tools -> Developer Tools -> Error Console would be very helpful.