General rule of thumb: Never send anything in E-Mail that you don't want to appear on the front page of a newspaper the next day.
E-Mail is stored unencrypted on several servers along its path, and due to some weird legislation is required to be stored that way for several years ( in case authorities "need to know" what you said ). Also, any one of those server's administrators could be bored and have bad ethical standards.
If you are sending unencrypted personal E-Mail from work, there is no doubt that your employers can and probably do read some of your mail.