About half of it comes from MailChimp. The people at MailChimp will tell you, "we help our customers comply with spam laws and best practices." It's not true. They're spammers.
To understand that, we first need to define spam. I like the definition from Spamhaus.org. It is unsolicited bulk email. Bulk email is OK; I subscribe to several mailing lists. They send bulk email, and I get it because I want it. Unsolicited email is OK... in fact, it's even normal. An old friend recently sent me email inviting me to lunch. Great! It was unsolicited, but I was happy to get it.
It's Spam, Jim!Email is spam when it is both bulk and unsolicited. In the case of bulk mail, "unsolicited" means "the recipient has not verifiably granted deliberate, explicit, and still-revocable permission for it to be sent." Those words are from Spamhaus, and I couldn't say it any better. It's about consent. "Verifiably granted" means what is sometimes called the double-opt-in. The emailer sends you a message that says, in effect, "Thank you for signing up for our stuff. Click here to validate your email address." That is verifiable. That is what MailChimp doesn't do.
Don't Get Tarred with MailChimp's ReputationIf you are thinking of email marketing and you're thinking of hiring help, be careful not to get tarred with the reputation of a spammer. If people get spam from you, they'll naturally and correctly think you're sleazy. That's what will happen if you hire MailChimp. They send from several domain names, and every time I find one, I block it on my home email server. That mail "bounces;" it gets rejected without being delivered at all.
I also have an email address with one of the big, public email providers. I haven't figured out how to block entire domains there, but I do block the individual spammers, one by one. Their future messages get received and go into my spam folder. The sender paid MailChimp to send that message, but I never saw it. Don't let that happen to you!
I feel no obligation to "unsubscribe" from anything to which I didn't subscribe in the first place. In fact, I see a great advantage in not unsubscribing. It makes the sender's email campaign that much more expensive because the sender pays for those messages that go into my spam folder. Don't let that happen to you. I feel no obligation to make spamming cheaper or more cost effective. Besides, unsubscribing just confirms that the email address works.