Zendesk Mandrill pricing is divided into blocks (1 block = 25,000 emails). These blocks are sold on a monthly basis, at the end of each monthly cycle, the unused credits blocks expire. Based on your requirements, you purchase the number of blocks, the rate per block decreases as your email quota increases. If your volume of emails come between 1 to 500k emails, you will spend $20/block. Similarly, if it falls under 500k to 1M emails, the price per block reduces to $18/block.