RFC 8058 defines one-click as an HTTP POST from the mailbox provider to the URL in the List-Unsubscribe header, with the body List-Unsubscribe=One-Click and a Content-Type: application/x-www-form-urlencoded. The endpoint must return a 2xx and unsubscribe the reader without any further interaction. Gmail and Yahoo built one-click into the February 2024 bulk-sender rules — 5,000-a-day senders whose endpoint 5xx’s, times out, or requires a session cookie are treated as if the header were missing, and the message drops into spam.
The static validator tells you the headers are well-formed. This tool tells you the endpoint actually accepts the POST and returns a compliant response. The two are the necessary-and-sufficient pair — the header can be perfect and the endpoint still broken; the endpoint can respond correctly to a browser GET and refuse the POST. Nobody currently offers this test publicly.
This tool fires a real POST. Only test endpoints on domains you control or campaigns you legitimately received. Requests are rate-limited to five per hour per target host, and private / loopback / link-local IPs are refused before any request goes out.