Many mail servers lack support for SSL and SMTP Authentication. SpamFilter ISP supports both SSL and SMTP AUTH via Active Directory, LDAP, and Unix-style password files. If a user is authenticated, they will be able to bypass all filtering rules and use SpamFilter ISP as a relay to send their outgoing emails.
Administrators can then add support for SMTP Authentication (and SSL) if they have older mail servers that do not have these features.