Bulk senders at the large providers are expected to support one-click unsubscribe, and the implementation is a pair of headers plus an endpoint that actually works without a login.
The header pair
List-Unsubscribe carries the URL (or mailto) that a client can use. List-Unsubscribe-Post with the value List-Unsubscribe=One-Click tells the client it may POST instead of opening a page. The client sends a POST with that body to the URL, and the server must act on it immediately and answer 200.
What goes wrong
The URL points at a page that requires a session, so the POST gets a redirect to a login screen and the unsubscribe silently fails. The client reports the message as spam instead, which is worse for you than an unsubscribe. The endpoint must be public, idempotent, and fast.
Scope
One-click unsubscribe applies to the stream the message came from. If the same address also receives receipts, those must keep flowing — a blanket suppression breaks the product. Encode the stream in the signed token and honour it precisely.
The detail that keeps you compliant
Store the unsubscribe even if the message is later bounced, and make the suppression check part of the send path rather than a nightly job. A campaign that starts before the job runs sends to people who already opted out.
Volanea emits both headers per stream and applies suppression at delivery time, so a one-click unsubscribe takes effect on the next send.
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