Re: Change in unsubscribe links in single emails
So the email address that clicks on the mailto link is the one whoMore precisely, the email address in the From: of the message to the group's +unsubscribe address.
It would be up to the user's email interface to decide which address goes there - in a situation like Duane and mine, with multiple addresses managed through a single instance of Thunderbird. In a typical webmail setup, with only one email address ("account") in use, there's no ambiguity.
... and there would be no possibility that someone who forwards orCorrect.
Note though that some form of authentication is required with the +unsubscribe command. For Groups.io (and Yahoo Groups) this means that the system responds to the command by sending back a confirmation request email. The unsubscribe does not take effect until or unless it is confirmed.
That's a big downside - users often miss the confirmation email and become frustrated that the unsubscribe didn't work.
I think that downside could be mitigated, in most but not all cases, by using email authentication (such as SPF) as a "good enough" confirmation so that the system can send back an acknowledgment rather than a confirmation request.