One-click unsubscribe headers lower spam-complaint rates compared with a footer link alone

Hypothesis

When a mail client can act on RFC 8058 headers (List-Unsubscribe with an HTTPS URI and List-Unsubscribe-Post: List-Unsubscribe=One-Click, both covered by DKIM), recipients who want to stop receiving a list choose the client's unsubscribe control more often than the "report spam" control, so the complaint rate per delivered message is lower than for the same messages sent with only a footer link.

Prediction

Splitting one list at random into a one-click arm and a link-only arm over several sends, the one-click arm shows a higher unsubscribe rate and a lower complaint rate, with the sum of the two roughly constant; the difference is largest at mailbox providers whose clients render the header control. Google's sender guidelines require one-click unsubscribe for bulk senders' marketing and subscribed messages and ask senders to keep the spam rate reported in Postmaster Tools below 0.30%, with 0.10% as the level to stay under, so the practical stake is the distance from those thresholds.

Proposed test

1. Randomise recipients (not individual sends) into two arms with identical content, footer link and sending infrastructure; the arms differ only in the two headers.
2. Run for at least four sends and record per arm: delivered messages, unsubscribes by channel (header POST, footer link, reply), complaints received through feedback loops, and the providers' spam-rate dashboards.
3. Compare complaint rates per delivered message with confidence intervals, and check whether total opt-out (unsubscribe plus complaint) differs between arms.

Status

No result is claimed. Confounders: receivers may already weight one-click support in their filtering, which would lower delivery rather than complaints in the link-only arm; and deliberately withholding one-click from bulk mail may itself breach provider requirements, so the test is proper only below bulk thresholds or with the receiver's knowledge.