Really? My attempts to get unsubscribed from a mailchip spam list were largely unsuccessful until I spammed as many mailchimp employees as I could find on LinkedIn. Mailchip offers zero support for those on the receiving end of their garbage.
Honestly, the most absurd part is that mailchimp is opt-out by default. I never once got any sort of "verify you'd like to receive messages from this sender" sort of e-mail address. Obviously mailchimp has no incentive to make their delivery opt-in because it would cut down on the number of messages delivered.
And, yes, you're right most of the alternatives are just as bad (e.g. Google, Sendgrid) from an end user point-of-view. I've had no regrets about blocking sendgrid and mailchimp.
Self-hosting my mail stuff means I'd see if I were bouncing legit email pretty easily.
I normally use the link in the X-Report-Abuse header. I invariably get an automated reply immediately (so if this isn't happening for you something must be going wrong; this seems to have been introduced about a year ago).
This is followed by what appears to be a semi-automated response (it varies depending on outcome but the text is usually made up of mostly the same content). I've had human responses from replying to that. (Though there is rarely the need—the initial complaint seems consistently to remove the complaining address from the mailing list, so that campaign stops unless the spammer manually adds the address back. Why doesn't everyone do this? It seems pretty obvious.)
I've also blocked Sendgrid as, despite the occasional response from their abuse contacts, the campaign doesn't stop coming. (I can't get away with blocking Mailchimp in any case as I have users relying on legit mailing lists that use it.)
> the most absurd part is that mailchimp is opt-out by default
I think this is understandable in terms of customers wanting to move mailing lists from one ESP to another without having to re-opt-in every subscriber. I wish there were a better way to manage permissions for mail receipt, but the SMTP infrastructure is made largely of despair and tears.