D. J. Bernstein
Internet mail
Internet mail message header format

Resent-* fields

There are many ways for a user to forward a message to another user:
     From: God@heaven.af.mil
     To: angels@heaven.af.mil
     Subject: take care of this guy, willya?

     > From: spamking@cyberpromo.com
     > To: God@heaven.af.mil
     > Subject: MAKE MONEY FAST
     > 
     > A once-in-a-lifetime opportunity!
822 specifies one mechanism, namely Resent-* fields:
     Resent-From: God@heaven.af.mil
     Resent-To: angels@heaven.af.mil
     Resent-Subject: take care of this guy, willya?
     From: spamking@cyperpromo.com
     To: God@heaven.af.mil
     Subject: MAKE MONEY FAST
     
     A once-in-a-lifetime opportunity!
This format effectively splices the new recipient into the old recipient list. Any 822 reader has immediate access to the original message. A reader that knows about Resent-* fields could provide a separate display of those fields, without the Resent- prefix, to show how the message was forwarded.

The new recipient might forward the message to someone else. 822bis allows multiple batches of Resent-* fields, treated the same way as Received fields: each new Resent-* field is added to the top of the message.

I recommend against using the Resent-* mechanism. For mailing lists it conveys no useful information. For manual forwarding it runs into practical problems: if the original Received fields are not stripped then the message often bumps into MTA Received-line limits; if the original Received fields are stripped then useful information has been lost. Furthermore, many users never see the Resent-* fields and are surprised to receive messages that were not addressed to them.