The SBS2003 Server Usage reports may show a large number of emails for a variety of reasons.
1. There is an acknowledge bug in SBS 2003 which is due to be fixed in SBS SP1 that can show this
2. The system may be being used as a SPAM relay host in which case
3, Your users may be actively involved in a number of online forums which communicate via email – Yahoogroups and MSN Groups are some such forums.
Check out the following MS KB Articles
Large number of messages in the SBS 2003 Usage Reports
How to block SMTP Open Relay in SBS 2003
*** UPDATE – This has also been seen as a bug which has been resolved in SBS2003 Service Pack 1 *** – Wayne – 13/6/05