"Header Field: From
Mailbox of message author
Specifies the author(s) of the message; that is, the mailbox(es)
of the person(s) or system(s) responsible for the writing of the
message. Defined as standard by RFC 822."
"Header Field: Sender
Mailbox of message sender
Specifies the mailbox of the agent responsible for the actual
transmission of the message. Defined as standard by RFC 822."
According to that, the 'From' field should be the mailbox of the author of the message, and the 'Sender' field should be the mailbox of the agent (in this case, a website) that is sending the mail.
See also, http://tools.ietf.org/html/rfc5322:
If that's right, then
is incorrect (though it might be what currently works best in practice).