Long before these issues because common, even before office email was common, I had to sign employment contracts that explicitly stated any works, inventions, and so on created by me on the premises during business hours using company facilities and equipment were the sole property of the company, with no exclusions. Even as a summer student this was an absolute requirement.
Any correspondence constitutes a piece of work, therefore by extrapolation those ancient contracts cover email written on company computers using company accounts. One could even legitimately extend this to emails written on company time, using a company computer, but sent from the employee's personal webmail.
Bottom line, if you are writing personal emails from work, expect there to be issues, and don't expect privacy.
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