Trouble with Ninja Forms and Mandrill

I'm having a website where a ninja form is placed. According to ninja forms it should work perfectly with wpMandrill plugin.

I have both installed and configured. Mandrill is working fine. Without ninja forms mails are successfully sent. I've also set up DNS records for mandrill so it sends on behalf of.

But I'm still not receiving any mails through ninja forms. I have actually no idea what the problem could be.

Has anyone any idea what this problem could be?

Thanks! :)

Sources: - http://ift.tt/1fE4CVI - http://ift.tt/1ltx7nt