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 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. The originator fields also provide the information required when replying to a message. When the "Reply-To:" field is present, it indicates the mailbox(es) to which the author of the message suggests that replies be sent. In the absence of the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field unless otherwise specified by the person composing the reply. -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers