That's why I never send personal email on the company's system. I also don't keep any personal files on the company supplied computer nor do web browsing on it. It's a hassle sometimes, especially when I need to carry around my personal laptop. And, in reverse, I never do "work" on my personal computers.
While I don't think my company is spying on me, I go by that assumption because they can start at any time without my knowledge. It's my way of mitigating that risk.
In general, I think it's also a good way to keep my personal life separate from work. I learned that years ago during some stress reduction workshops I participated in.