Exchange 2003 - Anti Spam Configuration

Exchange 2003 has some anti-spam functionality that can be implemented on
the server in a pinch:

Recipient Filtering
Sender Filtering
Intelligent Message Filtering (SCL Rating)
Connection Filtering (RBL Black lists)

These need to be configured and enabled on the SMTP virtual server before
it works.

This link will demostrate how to deal with an NDR type attack and shows how
to configure the filters inside the ESM
http://support.microsoft.com/kb/886208

http://support.microsoft.com/default.aspx?scid=kb;[LN];261087

Tarpitting is another mechanism used to prevent Directory Harvest attacks
http://support.microsoft.com/kb/842851/

If you need to dump the existing SMTP queue as it is filled with spam -
rename the Queue directory found here
http://support.microsoft.com/kb/822933