In 1986, the Federal government amended the Wiretap Act of 1968, extending the protection of information that is transmitted electronically to include email, telephone conversations, and data that is stored electronically: http://it.ojp.gov/default.aspx
While this act provides protection for the privacy of this information, it is still important to understand that the email we send from work using computers, servers, and electronic access that belongs to the company, is NOT our private property. It is the property of the company who owns the equipment and Internet subscriptions.
What this means, in light of the Electronic Communications Privacy Act, is that all information conveyed through company computers, whether or not it is business related, belongs to the company. It then becomes very important that we, as individuals, do not use company resources for personal use. Many companies have policies in place that govern the use of company resources for personal use. If you are not familiar with those policies, check with your HR department to find out what they are.
Deleting an email that may contain inflammatory information or information that could get the writer in trouble will not eliminate it. Email is stored on servers and is accessible by the company for many years. Instant messages, on the other hand, may not be stored on servers, but many companies do choose to monitor and store IM messages as well. The bottom line is, do not use the company electronic resources to convey personal information or for personal reasons.