To send an email automatically to candidates when they apply, you can create an action. Here's how:
Go to the job you want to set up an automatic email for
Go to Job Setup on top right, then go to the Stages tab
There, click the Add an action button for the New Applications stage.
Choose Email, choose the email template you want to send, and save that action.
You're done! When a candidate applies through your job board, through email, or via an integration, they will get that email.
Keep in mind though that candidates that you add yourself inside enlist won't receive that email.