Filters can be applied as the Administrator that will affect all incoming mail to be screened for certain words that are known to be used in Spams that you are receiving. Examples are: Viagra, Rolex, etc. By setting up a filter for your domain you can reduce the number of incoming Spam messages while also being able to bypass Greylisting. (see above).
Postini is a third party mail filtering service offered by Google. It is highly effective in filtering all in-bound spam while also giving you additional control over what is being considered as spam, emails containing possible viruses, etc., by sending you a detailed report where you can allow messages to be sent if they are detected. You also have more controls over setting up SAFE SENDER lists.
For more information about adding Postini please go to the following link: http://www.google.com/a/help/intl/en/security/messagesecurity.html