An alternate, but maybe not more plausible explanation, is that the employee sent the email on the Google office network and the traffic was being logged anyway.
It is both legal and required by US law. If a company doesn't have a "system of record" that keeps track of all emails generated by employees, the company can lose their defenses against a wide array of possible legal troubles.
Your email is sitting on a machine that they have full permission to. Their EULA explicitly gives them permission to access your account (often for any reason).
Also typically you do those kind of correspondences from home.
