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[Archie]

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[chrisbrain]

Hi - It's a bit difficult to comment as there are many different SMTP implementations.

Do you have an Azure admin you can contact who manages your accounts? This came up as the first google result for that error message:

https://docs.microsoft.com/en-us/answers/questions/512954/535-57139-authentication-unsuccessful.html

And looks like it might be relevant.