Mandrill credits are sold in blocks of 25,000 emails. At the end of each billing cycle, unused credit blocks expire. Your pricing depends on how many blocks you purchase at the beginning of a month. And since we build discounts into our payment structure, your per-email pricing decreases as you send more email.
These pricing changes will take effect when you connect your Mandrill account with a paid, monthly Mailchimp account.
Our Knowledge Base includes answers to frequently asked questions about Mandrill and Mailchimp.