This question doesn't fit the site (off-topic; doesn't ask a programming question; more of tech support for another site) - but while you are here, here's the answer you want. Again, you should contact them for this kind of question. Please read https://mailchimp.com/resources/guides/common-rookie-mistakes/html/
Mistake: Confusing transactional emails with email marketing
Do you have a list of customers who have purchased products from your
e-commerce store? They’ll probably expect receipts and shipping
notifications via email. Those types of one-to-one messages are called
transactional emails, and they're different from email marketing.
That’s why we developed Mandrill. Mandrill is a delivery API for
MailChimp users who want to send transactional emails. Mandrill is
optimized for emails like password reminders, order confirmations,
receipts, and personalized notifications, and will help you manage and
monitor your transactional messaging through advanced tagging,
webhooks, and more.
As they say, MailChimp is not for transactional emails. Mandrill is.