Mandrill is not "the backend service behind MailChimp". Mandrill came after MailChimp and is a separate thing - they only allow it to be used for transactional email. Sending a newsletter via Mandrill violates its Terms of Service.

I just went through Mandrill's TOS [0] and didn't see anything referencing a ban on emailing lists.

From my understanding, and having used Mandrill for some time, it can be used for anything you want (that follows its double opt-in, no spam requirements).

[0] https://mandrill.com/terms/

Just to back this person up, on Mandrills account page it states:

"Use Mandrill to send automated one-to-one email like password resets and welcome messages, as well as marketing emails and customized newsletters."

Emphasis on the "as well as marketing emails and customized newsletters" section there.

https://mandrill.com/about/

The difference of Mandril is that is doesn't have all the fancy features of Mailchip that people "would expect" to create a newsletter etc so Mandril is aimed at developers/software etc. That does not mean you can not use it for same uses as Mailchimp though.

It's been 3 years, but there was word from MailChimp that that is ok: "Yes - you can use Mandrill for newsletters"

https://news.ycombinator.com/item?id=4132375