How to set transactional email api key for external app in drupal like mandrill Like mandril,I have app for routing system transactional mails through that.I got create account in that app and getting api key for my drupal site,But i dont know how to process it (where to add that api key to configure it)on drupal to send mail.please clarify.
Assuming you're using the Mandrill module, the API key can be configured at this path: 

/admin/config/services/mandrill