Unsubscribe Get

The link in the mail. Renders a one-button confirmation, writes NOTHING.

A GET here must not change state. The recipient's own mail client is not the only thing that follows this URL: Outlook SafeLinks, Proofpoint URL Defense, Mimecast, Barracuda and Gmail's link prefetch all issue a GET on every link in a message before a human has seen it, and several do it on delivery. A GET that wrote the sentinel would therefore unsubscribe customers who never clicked, silently, on the very campaigns this change adds the header to, and nothing in this repo ever clears the sentinel again. That is why RFC 8058 puts one-click on POST.

The button below posts to the same URL, which is also the target the mail client uses for List-Unsubscribe-Post. One code path, one write.