What is one-click unsubscribe?
One-click unsubscribe lets a mail client unsubscribe the reader with a single HTTPS POST, without opening a web page. RFC 8058 defines it: a List-Unsubscribe field with an HTTPS address, a List-Unsubscribe-Post field reading List-Unsubscribe=One-Click, and a DKIM signature covering both. Gmail requires it on bulk senders' marketing mail; Yahoo requires a List-Unsubscribe field and highly recommends the one-click POST.
Reading the result
Paste the headers of a message you sent, copied from a mailbox it reached. Each of RFC 8058's three requirements is marked as met or missing, then come the addresses in List-Unsubscribe and every DKIM signature with the fields its h= tag covers. Whether a signature verified is read from the receiver's own Authentication-Results, so headers from your Sent folder show it as not confirmed.
Common problems it finds
A mailto address only
Google's sender FAQ says mailto links do not meet its one-click requirement: the field needs one HTTPS URL. Yahoo's sender best practices accept mailto and highly recommend the RFC 8058 POST.
The fields are not signed
RFC 8058 §4 needs a DKIM signature whose h= tag names both List-Unsubscribe and List-Unsubscribe-Post, and says a receiver should not offer one-click without it. Many platforms add the fields to the signature only once one-click is switched on.
An address that opens a page
The POST itself must do the unsubscribing. Google says a one-click address that leads to a landing page does not comply with RFC 8058, and the RFC forbids answering the POST with a redirect.
Unsubscribes honoured late
Yahoo says an unsubscribe not honoured within 2 days fails its requirement, and Google recommends 48 hours. Both rules cover marketing and subscribed mail; transactional messages such as password resets are excluded. Google's covers senders of close to 5,000 messages a day or more to personal Gmail accounts.