Gmail allows a subscriber of an external mail list to press a button
that says UNSUBSCRIBE which is adjacent to the FROM: field displayed
in the message.
Strangely, this button does not appear for LISTSERV distributed
messages even though the RFC822 header is there.
In the first example, a non-LISTSERV message distributed by
Starbucks had the UNSUBSCRIBE button, but in the second example
(which was distributed by LISTSERV), no such button was displayed: