When you open your FrontPage form mail, you will notice a default "From" address. This is because the form submission is actually coming from your site, rather than the person who fills out the form. Although your host or server admin may be able to set up something a little more user friendly, it will still be a static from address.
The real meat of this issue is to have a way to respond to your site users. This is done with Reply to:
The way to get the users email in the Reply to line is to follow the method below.
Right click inside the Form proper (the dashed or dotted line around your form).
Select "Form Properties".
Then "E-mail Results" tab.
Under "Email message header:" "Reply-to line:" check "Form field name" and type the field name for your user email exactly as it appears in Form field properties.
Save and publish.
Now when the email is opened, the form will still be sent by the site, but the Reply to will be the users email.