It depends on what mail server your company is running. I have adminstered Lotus Domino servers for some companies. Domino has a journalling facility which keeps a copy of every inbound and outbound email. Companies will use this for their own protection. In the UK a company can be liable for the emails sent from their server by one of their employees. They therefore need to access emails that may have been deleted from an individuals mail box. The volume of traffic generally means that it is only used to find specific emails rather than all traffic being monitored and read. Always send mail assuming that mail can be read.
I always advise people to have a personal email (hotmail, googlemail, yahoo or what ever). Don't use the company email for personal messages.
Your employer should have an internal IT policy that says what you may use company IT for and what they monitor.
Many companies also track which websites you visit.
My golden rule is never write anything that you would not be prepared to hear read out in public. Don't visit websites from company or other peoples pc's that you don't want people to know that visit.