Mail is getting rejected when sent using Mandrill API

I am trying to send email using Mandrill API, but I am getting response as below:

{ "email":"_emailid_", "status":"rejected", "_id":"967859c539b1416abc2613b5a3619a46", "reject_reason":"invalid-sender" }

I am sending the mail by hitting curl call on mandrill api like here.

Can somebody tell me what might be reason for the rejection ?

The good news is that your API key is fine. The bad news is that you probably need to configure the email sender domain so that mandrill knows the sender is valid.

It's been a long time this question was posted, but I guess I still can contribute.

I had the same issue this week when using MailChimp Transactional API calling a POST method through the command curl -X POST
https://mandrillapp.com/api/1.0/messages/send-template
-d '{"key":"","template_name":"","template_content":[],"message": etc... }

and I got the error message :

[{"email":"[email protected]","status":"rejected","_id":"2bff316436814a399fafea22fda4a4dd","reject_reason":"recipient-domain-mismatch"}]

Asking MailChimp's support team about this, they reply , after a couple of days, the following :

The recipient domain mismatch reject reason is specific to demo and trial accounts and refers to a limitation in those accounts. Sending is restricted to the domains you have set up in the account already. This means your recipient would need to be on a domain that you have the same setup as mentioned for your sending address. If you change to a paid account, this restriction would be removed automatically and you could send to any recipients. Plainly put, you can only send to domains you have verified/authenticated on the demo mode.

I use a free account. I search the documentation and several other sources in the internet but I found no reference to this free account limitation. It would be very productive if such limitation was explicitly put.