Your question is clear.
But here are the possible solutions:
If you are talking about an auto reply to the sender, you can turn on the auto reply option in any email service provider like Gmail, yahoo, hotmail etc. When someone sends you a message they will receive an automated reply, something like [you message has been received].
If you are concern about replying manually to your senders, it is already done by muse,
When you will receive an email from your website, you can directly reply to that email and the email will be delivered to the actual sender. its a built in feature in muse. (see screenshot)
So if you just reply the email (where you received it) the email will be delivered to your sender.
Hope this helps.
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Thanks for your prompt answer. I have made a muse email form from the default widget and I want this form to automatically send an email to me and also automatically send the same email to the person who fills the form (thus the person who writes his/her email in the form).
You can do that from the email service you are using such as Gmail, Yahoo etc.
Which email service you are using?