Every server that handles your message adds some information to the message headers. At a minimum, it will add a Received: header, which indicates when and how it handled the message. But it can also add other information.
At Pobox, we add several headers. They show you the information we used to spam filter the message, indicate why we took an action on a message if we filtered it, and help us debug problems. Every message Pobox handles will have the following headers added (in addition to the Received headers which show the servers that handled the message):
Messages that are sent from Trusted Senders will also have the following header:
X-Pobox-Pass: <email address> is whitelisted
Messages that are sent to Pobox Plus and Mailstore accounts with filters will also see the following headers:
Messages that are actually filtered will also have these headers:
Messages sent to Mailstore accounts will have: