Authentication failures on initial Exchange backup

The first backup for a newly-created Microsoft 365 Exchange user fails. The specific failure states vary, but consistently relate to authentication.

Upon investigation, Technical Support may find "ErrorAccessDenied" messages in the server logs.

Environment

The new user has never logged into the protected Exchange service before, preventing necessary processes from completing the account setup.

The new user must log in to the Microsoft 365 user account at least once before backups can complete successfully.

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