Thanks a lot for your comments. It's figured out to be an issue with auto-forwarding function set by the user.
He set up forwarding function to forward all the emails to another one which is outside our organization, and unluckily, that account was disabled last week, which caused this issue:(
It turns out that Microsoft bounced email was not that SMART, it's not this user account not found, it was another one (forwarding to) being disabled.
Thanks again for your time!