Many organizations use MailChimp, SendGrid or SocketLabs email integrations to ensure their emails are delivered with whitelisting and lower load on their servers. Switching to Mandrill can dramatically reduce the cost of these services.
Mandrill is a service provided by MailChimp that uses the same email delivery infrastructure as their well-known service designed for delivery to newsletter subscriber lists. Mandrill's pricing is based entirely on number of emails sent, rather than a combination of subscribers and email volume.