The plugin lets you use “Mandrill” system to manage emails. In order to do that, activate Mandrill in YITH Plugins > Review for Discounts > Mandrill Settings and add the requested API Key.
To find the API Key, log in to your account and go to this page: https://mandrillapp.com/settings/index. Go to the “SMTP & API Info” section.
Click on “New API Key” to generate a new API Key.