It's a reasonable idea for mailing list software to put the list in
Sender given that it's the "agent responsible for the actual
transmission of the message" as RFC2822 specifies. But you could just
as easily argue that the list is just relaying the message and the
agent actually transmitting the message is the webmail provider the
user used to compose the message. In a case like mine where I send it
from gmail but use my .edu email address that's a sensible
interpretation.
But my point was that while the RFC says what to put there there's
absolutely no reference anywhere for when the information should cause
any MUA or MTA to behave differently. Responses are governed entirely
by From and Reply-To, bounces don't use it, and so on. It's a bit of
extra information that can optionally be added but there's no
indication of how it should be used. Using it for anything aside from
abuse investigations is going to pretty much always be wrong.
RFC2822:
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
appearin the "Sender:" field and the mailbox of the actual author would appear in the "From:" field. If the
originatorof 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.
The originator fields also provide the information required when replying to a message. When the "Reply-To:" field
ispresent, it indicates the mailbox(es) to which the author of the message suggests that replies be sent. In the
absenceof the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field
unlessotherwise specified by the person composing the reply.
