I don't have actual numbers, but a cursory examination of my Gmail spam box says spam senders don't generally provide a valid List-Unsubscribe header. One of Gmail's spam triggers may in fact be, someone clicked on the List-Unsubscribe header and the FBL message bounced.
Tracking image loads and clicks would be a much more effective method of determining that an address is valid for both legitimate mass emails and spam.
Only if you've previously exchanged messages with them.
How many people do you think load images versus click Report Spam on an ISP with a feedback loop? And which group do you think would be a better target for future messages?
There are laws regulating e-mail solicitations. People can be sued for SPAM, and incur criminal charges. Basically, if any mail provider notices that lots of its users complain about SPAM, they have legal standing to bring that person before a judge. Sending out a warning of possible legal repercussions is the first step.
Presumably, the spammer is contacted via e-mail with a strongly worded message, since it's very clear that they have the ability to interact via e-mail.
In theory, you as an individual, could run your own mail server, and if you received spam, you could threaten to sue or press charges. At some point before you actually do that though, you'd probably try to contact the person first. When cmwelsh uses the word "ping", that's just a euphemism for making contact, and not a specific technical term of any kind.
While that's sort of correct, there actually is a technical "ping" called an FBL message that gets sent by the big providers when someone clicks the "report spam" button.
I don't work for Mailchimp, but it isn't that complication: Feedback loops with major email providers, monitoring bounes and engagement rates, and (of course) tracking abuse complaints. If you're sending out bona fide spam, many people will take the time to send a message to abuse@yourprovider.com
Among the consumer-grade ESPs, Mailchimp has a reputation for being extremely strict about anti-spam enforcement.
