Every email that is sent from a valid user in your system, can be assumed is sent to a safe, valid email address. Wouldnt it be helpful to be able to mark all incoming email as being from a "valid" email address. The concept of a list of safe/valid email addresses is called a whitelist.
Auto Whitelist, records the "To" email addresses for all outoing emails (for pre defined email accounts), each of these addresses are added to the whitelist. All incoming email is then marked with a header indicating if the sender is on the whitelist.
Auto Whitelist, is a postfix content filter. It runs in memory as a daem on, and postfix can be configured to pass all mail through Auto Whitelist.