According to a Google representative, they are preparing to include Postini as a standard email service to control spam, and will be migrating users for eDiscovery features to Google Vault. See http://blogs.wsj.com/cio/2012/08/21/google-prepares-customers-for-postini-shutdown/.
Gmail includes a free tool (https://tools.google.com/
Stay tuned, as we’ll have more to report about Google Vault in future blog posts as we put together more information about the Gmail email format and eDiscovery options.