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Exchange SMTP Auth Fails with: 451 4.7.0 Temporary server error. Please try again later. PRX5
Office 365 refuses SMTP authentication with error: 451 4.7.0 Temporary server error. Please try again later. PRX5 This is when accessing External Send SMTP AUTH through OAUTH2 authentication. Initia...
Thank you very much for your post, I have (almost) the same problem.
I say "almost" because in my case it's not a global admin account, but after some trial & error definitely it seems that it is related to what you are describing.
You said that this bug has been publicly noted. Do you have any additional reference?