To sign up for Postini please visit: http://www.fsr.com/Spam/
Read the information on the page then click the "Sign up" link near the bottom.
Read the agreement then click the "Continue" button at the bottom.
Enter your username and password in the spaces provided and click "Continue".
Click the "View/Add Services" button.
Use the drop-down under "Add Additional Service:" and click "Postini email filtering..."
Click the "Add selected Item" button once (If you click it more than once it will add the service more than once as well). Once the service is added allow up to two business days for the setup to be completed. You will receive an email from Postini when your account is set up with instructions on how to configure the filter.