Detailed information on blocking or allowing traffic from a specific IP can be found in the Administration Guide:
http://www.postini.com/admindoc/conn_manual.html
Note: This article is intended for administrators using Message Filtering, Message Security and Message Discovery. If you're using another edition, your service may include different features from those described in this article.
For a checklist of security best practices and links to other important resources for your service, visit Postini Security Central.