I'm sorry to say that this plugin no longer supports the new mandrill method, as in madrill, we must verify the domain now, and if the "Sender" is the email posted at the form, it will add those domains, and attempt to verify them. Thus refusing to send emails. Here's a print of example: http://i.imgur.com/EnmA6J0.png

The email is gmail, and it will add it in the domains because the sender: http://i.imgur.com/fAZRaXp.png Is Gmail.

I hope you can change that from the email in the form, to the email in the settings.

But, overall, good plugin ;)