Sorry but this is not a feature of the contact form included with mojoPortal so you would have to implement a custom solution to do that.

To me it is a problematic thing to do, my advice would be to just reply to the person who submits the form.

The reason I see it as a problem is that then someone could post an unsavory message and put in the email of a different person instead of their own and now your site is sending unsavory messages to people and it can hurt your reputation. There is no way to be sure the person submitting the form is entering their own email address. The person who submitted the form already knows what they submitted so this additional danger of sending bad spam to strangers is better avoided in my opinion.

Best,

Joe