List-Unsubscribe-Post
The header field (RFC 8058) that turns a List-Unsubscribe https link into a one-click unsubscribe; its only allowed value is List-Unsubscribe=One-Click.
What it means
Some software fetches the links in a message, for example to scan them, and plain GET requests to unsubscribe links were unsubscribing people by accident. RFC 8058 solves this by having the mail program send an HTTPS POST instead, with List-Unsubscribe=One-Click as the body.
The rules are strict. The header must contain exactly that key and value, the List-Unsubscribe field must include an https URI, and the message must carry a valid DKIM signature covering both headers. The URI must identify the recipient and the list by itself, since the POST carries nothing else and must not include cookies or authorisation; the sender's server must not answer with a redirect; and the mail program must not send the POST without the user's consent.
Example
List-Unsubscribe-Post: List-Unsubscribe=One-ClickRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
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