email to send back automatically te sender

You need to establish Mail Rules through the preferences for Mail. These rules are run manually on the selection of email.

There is little use in auto replying to emails. It only serves to verify that the sender has reached a working email address. And you run the risk of getting into a race condition. If the sender also sets up a similar procedure to auto reply to your emails then you will quickly get into the situation where emails are flooding the system just going back and forth between the two of you.

What is the purpose of auto replying/sending an email back to the sender? Maybe there is a better way to meet your goal.