Specify a custom reply to email address for the email.
<input type="hidden" name="_replyto" value="email@example.com">
Instead of specifying a static custom "reply to" address, you can also choose to use an email address entered by the user of the form. In order to do this, give your custom "replyto_" field a value of "%" followed by the name of the field. Let's assume you have a field named "email" which value you'd like to use for your custom "reply to" field, then you'd give you "reply to" field a value of "%email".
<input type="hidden" name="_replyto" value="%email">