This is not about a problem with the current text, but rather a bit of confusion started by RFC 733 that might benefit from some clarifying language.
I see the confusion rear its head regularly and think it time to have some text that tries to help people understand the difference between information and its encoding...
Current:
3.6.2. Originator fields The originator fields indicate the mailbox(es) of the source of the message. The "From:" field 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. The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message. For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field. If the originator of the message can be indicated by a single mailbox and the author and transmitter are identical, the "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD appear.
Add after this:Note: The Sender (responsible agent) information is always present. The absence of the "Sender:" field merely means that the information is redundant with the "From:" field, so it is not redundantly encoded into a separate field. The absence of the "Sender:" field sometimes confuses readers into believing that the Sender responsible agent information has not been specified explicitly.
d/ -- Dave Crocker Brandenburg InternetWorking bbiw.net