In Gmail, you can set up automatic forwarding (useful when you change to using a new email address). E.g. If an email is received by account A, it will be automatically forwarded to Account B.
Firstly, how do you detect if the email in Account B was sent directly or sent from a forward?
Secondly, can this be done in series, e.g. Account A autofowards to Account B, which autofowards to Account C. What headers (if any) show the list of accounts the email has gone through? Or at least shows it was forwarded through more than one email account?