I got the reason, it was silly mistake. I was trying to send mail through my personal email id which is on different domain than to for which Mandrill is configured & verified.
Searching for the reason of error, I found that this error is sent from Mandrill when Mail sent from unverified domains or domains without valid SPF and DKIM records will be rejected with the reject_reason, unsigned.
For more information refer
- https://mandrillapp.com/api/docs/messages.html
- https://mandrill.zendesk.com/hc/en-us/articles/205582247-About-Domain-Verification
- https://mandrill.zendesk.com/hc/en-us/articles/205582267-About-SPF-and-DKIM
For doing required setting related to SPF & DKIM for Mandrill please refer:
https://mandrill.zendesk.com/hc/en-us/articles/205582277-How-do-I-add-DNS-records-for-my-sending-domains-