As an employee I prefer not to use the corporate network for truly personal email.
If I am the employer that responsibly monitors the traffic to and from our network, including TLS traffic, an employee that uses our network for personal use with a surveillance "tech" company service such as Google Mail, Facebook, etc. is putting her own privacy at risk. Because I can extract her cookies from the traffic, all she has to do is forget to log out once and I now have a "bearer token", i.e., a cookie, with no expiration,^1 that lets me access her account at any time in the future.
1 The type of cookie that lets users stay "logged in" indefinitely. A non-"tech" company with sufficient legitimate sources of revenue besides online ads may not use such cookies. For example, if an employee logs in to her personal bank account using the corporate network but forgets to log out, the bank website will log her out automatically, the cookies will expire.