Question : anti-spam for Exchange Server 2003

Hello,
I set up Exchange SErver 2003 in my home office.
What is an inexpensive way to filter junk email on the server?
I have used Postini in the past, but would like to try installing an inexpensive application on the server itself.
Any ideas?
SIMPLE - INEXPENSIVE -EASY_TO_IMPLEMENT that works, is what I want

Answer : anti-spam for Exchange Server 2003

>> if I try to add a duplicate of the clustered key, is there an elegant way to catch it ?

You could always create INSTEAD OF INSERT/UPDATE triggers and catch the duplicate (non-trivial) and provide a more friendly message or fail silently.