"New technologies make it possible for employers to monitor many aspects of their employees' jobs, especially on telephones, computer terminals, through electronic and voice mail, and when employees are using the Internet. Such monitoring is virtually unregulated. Therefore, unless company policy specifically states otherwise (and even this is not assured), your employer may listen, watch and read most of your workplace communications."
And, yes, my company stores all emails sent through their network. It's their property, not mine. They don't look at them, but they could. It's typically retained for legal reasons.