Google Postini Directory Sync Configuration Guide - Manual (Page 10)

10 Configuration Guide for Directory Sync
Directory Sync acts as a one-way synchronization tool. Your users and aliases in
the email protection service are added, moved or deleted, but your directory
server is not changed in any way. Directory Sync is launched manually from the
Administration Console in the email protection service.
Directory Sync is set up on a specific user organization, and synchronizes that
organization with an organization unit (OU) on your directory server. Directory
Sync can synchronize with one particular organization unit, or with a whole
subtree. It’s also possible to set up several organizations with Directory Sync.
Usually, each organization in the email protection service is set to map to a
distinct OU on your directory server.