When the Mail API receives an email, the body of the email becomes a note. Any note that the Mail API generates already started with the To and Cc email addresses of the inbound email. Henceforth, the subject of the inbound email is also displayed. The subject is presented at the very top of the new note.
This is a small first step that is part of a larger initiative that we are working on to make it possible for automation rules to work with the information that is included in the subject of inbound email messages.