Certains courriels publicitaires contiennent à la place du champ standardisé List-Unsubscribe (RFC2369) un champ X-List-Unsubscribe qui n'est pas standard :
https://tools.ietf.org/html/draft-costales-subscribe-01
(2) The X-List-Unsubscribe header, because it begins with an X-, is a
non-standardized header. Although some software may recognize it as
specifying an address for unsubscription, other software will not.
The leading X- prevents any requirement that all software recognize
its special meaning.
Alors que les champs List-Unsubscribe contiennent en général un lien <mailto:>
et/ou un lien <http(s):>
(…)