When i= was considered important in DKIM, many people considered it a
valid way to verify the identity of the sender of a message, given that
*) it was actually used
*) it really did map into the name used with the From: header
So the text about "belongs to the person who actually sent the message"
could be considered a reference to the use of i=.
Well, yes, and no. In retrospect, that was always a failure of
communication. The i= bit came from people in corporate environments
where the mail system is locked down, and you can't put anyone's
return address but your own on your mail. But, of course, there are a
lot of mail systems, and only some of them are like that.
ietf-smtp mailing list