According to G Clueley, Mailchimp has jumped the shark

https://www.grahamcluley.com/can-no-longer-recommend-mailchimp/

Apparently until recently they were defaulting to double-opt-in, asking the recipients of newsletters  whether they had really signed up for a service, and were, well, kinda preachy about it. (Good!)

Inexplicably, they have moved to single opt-in where anyone can sign up anyone else for services provided by Mailchimp.

Also inexplicably, they are going against the GDPR for mailing to europeans, and have no clue about it.