MailChimp seems like a good idea from the sender's view, but it's a nightmare on the recipient's end. I've used MailChimp as both a sender and a recipient.

As a sender, MailChimp lets you upload/add a list of email addresses you want to send to, so you can fire off an email from MailChimp and it will automatically email all of the recipients on that list. What MailChimp doesn't do is perform maintainance on your list. That is up to the sender. The big issue is that if you're not a large company, and you're just sending out a newsletter or something, some people may hit the reply button and ask to be removed, and you may or may not receive the email with the request. It pisses people off, especially the non-tech-savvy folks who can't figure out how to get your emails to stop. You'd have to include your own unsubscribe link in the email, and have a server to capture these requests to either remove them manually, or have a script do it automatically.

As a recipient, MailChimp is one of the largest email distribution services used by spammers. (another notable one is SendGrid). MailChimp sucks when trying to contact them for spam complaints. If I forward the spam email to their abuse address, they ignore it and the spam keeps coming. If I fill out their webform, I get an automated response, and then a 50/50 chance of whether they'll actually crack down.

Eventually MailChimp does crack down because they want to keep their mail servers off the public spam blocklists (like Spamhaus). I think Lucky's problem was that either he/his team either didn't spend enough time (if any at all) purging emails when people unsubscribe. You know how when you unsubscribe from an email list, they give you some absurd "Please allow 7-10 days to take effect" message? That is basically a courtesy request for time for the webmaster/list manager to purge your email from the list manually. People are NOT obligated to honor the 7-10 days (the CAN-SPAM Act does not require this), and can start reporting all future emails from them as spam. MailChimp, despite their lethargic response, will usually crack down (whether the sender is sending legitimate emails or not) if they receive multiple complaints as it's in their best interest to stay off public spam blocklists.