I delete all emails, and recommend others do the same... That is better than nothing, but doesn't mean that the system admin of the email server can't see your email.
It's trivial to make a copy of any mail that hits the server and store it elsewhere.
Exactly system admins can get into anyone's account. I was blessed (sarc) straight out of college to work for a jerk that regularly had the company's system admin read people's email in this particular corporation to get advantage over others who were in competition with him in the company.
I guess people know now how visible just about anything they do is this day, but not so much back then.