Mailchimp/Mandrill have specially bad customer support and business user experience. At one point they closed the account of a company I was working at without any previous notice and without any recourse. They just sent a blanket email "Your account has been suspended". They left us scrambling for an alternative (we had both transactional and marketing email). We happily migrated to SES for transactional email and SendGrid for marketing.
I no longer recommend or use Mailchimp.