Mandrill is an email infrastructure service designed to help applications or websites that need to send transactional email like password resets, order confirmations, and welcome messages.
Any bulk email should be sent through MailChimp, rather than Mandrill.
View our Acceptable Use Policy for additional details.
You'll need to add SPF and DKIM records and verify ownership of your sending domains before you can send email through your account. Mandrill will not send any email from unverified domains or domains without valid SPF and DKIM records, including public domains like gmail.com, yahoo.com, and more.
A message that is rejected with the reject reason unsigned indicates that the sending domain hasn't been properly set up, and that your account is unable to send and authenticate email from that domain.