I send an important message to a group of people and I would like to have their replies automatically filed in a separate folder.

Can I create a sorting rule that will filter the replies based on a unique identifier in the original message header?  I don't want to filter replies based on the Subject line, because people will change that.  I know such a header must exist because the Bat has a magical way of searching my folders for the reply to a specific message.  How does it know?