Any email sent to email addresses hosted on Google Apps can be filtered with Postini (feature from GoogleApps Business Edition). Postini is the first line of defense from spam and viruses. An e-mail may be quarantined by this system if it contains a virus, is clearly identified as spam or violates system policy rules. If an e-mail is flagged by the Postini system it will be added to the Quarantine area and user will be alerted in a daily Quarantine Summary. The Quarantine Summary has a Deliver option for each message, so if a message has been placed in Quarantine inadvertently a user can request its delivery using this function. If an expected e-mail is missing or a user just wants to check to be proactive, Quarantine can be checked in real time by logging into Postini. From this location, the Quarantine box can be viewed and messages can be delivered if the user so chooses.
Generally, Postini automatically create user accounts (along with any associated alias addresses) by synchronizing account information with Google Apps account. The first synchronization occurs when service with Google Apps is registered, as part of the activation process. After that, the service synchronizes accounts approximately every 15 to 30 minutes, so the new users quickly receive all the benefits of Postini (Message Security or Message Discovery)
Postini service stores a list of users, to allow each user to have custom quarantine and mail filter settings. With Directory Sync, the email protection service can contact the Active Directory(AD) to collect the user list information. Directory Sync will add, delete or move users so that the registered users in the email protection service match the registered users on AD. Directory Sync connects to AD securely, using standard XML technology. Some setup is required to enable Directory Sync to connect to AD