Legal Question in Employment Law in California

If it is on company computer equipment or systems or email accounts and servers, etc., they can do anything they like, It is their property, not yours. If your personal use was on company time, that is misconduct for which you can be disciplined or fired. If it is a purely personal computer at your home with a personal email account, unrelated to the employer, that would be an invasion of privacy among other things. I doubt they 'hacked' into your personal computer at home; If you can prove they did, feel free to contact me. I suspect you mean they looked at personal email sent/received from a company computer.