rfc1036 December 1987

2.2.2.  Sender

    This field is present only if the submitter manually enters a "From"
    line.  It is intended to record the entity responsible for
    submitting the message to the network.  It should be verified by the
    software at the submitting host.
    For example, if John Smith is visiting CCA and wishes to post a
    message to the network, using friend Sarah Jones' account, the
    message might read:

    From: smith@ucbvax.Berkeley.EDU (John Smith)
    Sender: jones@cca.COM (Sarah Jones)

    If a gateway program enters a mail message into the network at host
    unix.SRI.COM, the lines might read:

    From: John.Doe@A.CS.CMU.EDU
    Sender: network@unix.SRI.COM

    The primary purpose of this field is to be able to track down
    messages to determine how they were entered into the network.  The
    full name may be optionally given, in parentheses, as in the "From"
    line.