Are you talking about from a workplace or your home? If it's from work then yea, even though you delete them, the mail server can still have a copy. If it's from home, it depends on what kind of email you're using. I imagine they could if they were already watching you. If it's something like hotmail or yahoo just use something to clean your tracks. I guess if you were being watched they could hold your emails as well. Either way I don't think you have to worry about it. Paranoia is good to a point but don't let this get to you