Can My Boss Read My Mail?
SUMMARY: Be careful what you write in e-mail from your office.
The short answer is yes if you are using your company's electronic mail server.
Many companies now have acceptable use standards for email. These tend to include lines such as "The Company has the right to read any electronic mail passed through the system." This does not necessarily mean that your boss is personally going to snoop and look at every message sent, but it means they have the right to do so. Most of the time, mail is scanned to ensure harassing or harmful material is not passed through the system, viruses don't get sent, pyramid schemes are not started at work, etc., but there are other reasons mail may be read.
Just because you delete a message on your system does not mean it is gone forever! Backups of email messages can and sometimes are automatically made on the server level for legal reasons.
So, play it safe. Don't send any mail at work you wouldn't want others to see.