I was expecting the self-hosted spam cannon service that also went by the name kaboom a few years ago. I assume anyone who hosts a mailman list server is aware of this nuisance by now. Someone had created a list of thousands of mailman mailing lists across the internet and had a simple javascript page that could then be loaded and a victims email address entered. You'd select how many emails they should receive and hit run (increments in thousands). The script would then deep link to initiate a subscription request for the victim. All the victim would receive were requests for them to click the included link to activate the subscription, so they didn't actually get subscribed. But it would generate thousands of unique one-off emails to their inbox. The fix for list operators was to block the deep link into the subscribe url of course, and it's been several years since I've seen kaboom in my referral logs at this point.