Resent fields SHOULD be added to any message that is reintroduced by
a user into the transport system. A separate set of resent fields
SHOULD be added each time this is done.
For instance, the "Resent-Sender:" field corresponds to
the "Sender:" field
RFC 5322 3.6.6:
Resent fields SHOULD be added to any message that is reintroduced by a user into the transport system. A separate set of resent fields SHOULD be added each time this is done.
For instance, the "Resent-Sender:" field corresponds to the "Sender:" field