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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...
I can confirm that the issue is related to the user belonging to some roles. After removing all the roles my user was able to send messages through SMTP.
I still have to add the roles back one by one to catch the culprit. For now I can only say that it's not related to the Exchange Admin Role.
Odd, i don't have any roles for the user i am testing..