Hello,
It's not possible to know for sure.
The message appears to have been generated by your Microsoft Exchange server.
A larger company will sometimes have a process that would require a manager to get HR approval before they can get access to an employees email.
At a smaller company, my best guess is to think about what kind of culture do you work in?
If it seems like a reasonable place I doubt anyone is watching, and if your manager did get an alert, hopefully it's something that can be discussed.
On the other hand, if you feel like someone is always out to get you, and it's that kind of place, maybe they already got an alert or made IT give them access and are always reading your email anyways.
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