How many users and how much mail do you handle? You might be better off using a third party vendor like Message Labs or something if is a large volume of mail.
For Exchange 2003, there is nothing built-in. You would need to purchase a 3rd party product either to be installed on the Exchange Server itself or off the box. If it’s off the box, then you would need to configure your SMTP connector on your Exchange to forward to “smart host” which would be this 3rd party product. Most commercial antivirus products (on-box or off-box) would provide you content filtering for which you want to use.