[jacktoole1]

Maybe more importantly, they would know whether their message was classified as spam (or a pretty good heuristic thereof). A naive bayes classifier for spam relies on the spammer not knowing whether the message was marked as spam or not. Such a classifier isn't difficult to fool if you can test how a given message is classified. (Presumably gmail's spam filter is more advanced these days, but the idea that it is easier to fool if you can tell the result still seems reasonable.)

[derefr]

Here's an alternative idea: what if clients would only honor rel='unsubscribe' links with an HTTPS URL scheme, and only finish the TLS handshake for those requests if the host sends the client a valid Extended-Validation certificate?

Every spammer who wanted to "trick" the auto-unsub mechanism would basically have to first dox themselves for all the world to see. And any certificate that turned out to not be a valid means of contacting the spammer would be quickly revoked.

[Houshalter]

Well can't they do that anyways, by sending emails to an address they own and seeing what gets classified as spam?