Paul Smith says...
To me, a suitable spec would be something like:
List-Unsubscribe
-----------------
Syntax:
List-Unsubscribe: "<"URL">"[,"<"URL">"]...
where URL is the URL to go to to unsubscribe from the mailing list.
Example
List-Unsubscribe: <mailto:ietf-822-request(_at_)imc(_dot_)org?body=unsubscribe>
That's it.
Sure, that's it for that one.
Now write a spec for the "Sender" header field.
Be sure to include when an MUA SHOULD include it, when it MUST, whether
it ever MAY or MUST NOT. Be sure to cover if and when it's OK for a
mail submission server to add it on behalf of the MUA. Be sure to
include how it relates to the "From" and the "Reply-To" fields. Be
sure to relate it to mailing lists, as well as to regular e-mail. Be
sure to cover what the field is used for, and when it's appropriate to
use "Sender" as the recipient of some sort of return message. Be sure
to make all this clear enough that no one gets it wrong when they
implement it.
And then look at all the cases where it's been done wrong over the
years (and understand why X-Sender exists). Some fields are easy to
explain, easy to implement right, and have been a smooth sail.
Some have capsized, and the Coast Guard is still unable to find any
survivors.
Barry Leiba, Internet Messaging Technology
(leiba(_at_)watson(_dot_)ibm(_dot_)com)
http://www.research.ibm.com/people/l/leiba