I'm not sure if you have seen the following blog posts:

https://wordtothewise.com/2016/08/subscription-bombing-esps-spamhaus/ https://wordtothewise.com/2016/08/spamhaus-comments-on-subscription-attack/ https://wordtothewise.com/2016/08/ongoing-subscription-attack/

While mailman does double opt-in, one can still fill a mailbox with account confirmations, what are the methods to stop a bot submitting email addresses for registration across several lists?