How are we creating new users?
When you create a new user in Windows Small Business Server 2003, the Add User Wizard creates an Exchange mailbox for that user and adds the user to the default Exchange distribution group. You can also use the Change User Permissions Wizard to create Exchange mailboxes for users.
Are we doing LDAP or Directory lookups?
Exchanges spam filter checks to see if your users actually exist before accepting the mail – if the LDAP isn’t updated, then it has no idea that the new user even exists.
What options so you have in place for Spam or Antivirus filtering?