reply in gmail

I have a question about reply in Gmail

Everything works fine the form send perfect the emails to my account.. the confirmation mail sends perfectly to the person who filled in the form.
My Gmail gets all the input perfectly. no problem…
But when I want to reply that mail in Gmail it goes wrong… The sender seems to be the website so I can’t reply to that incoming mail. When I try to set the from to your e-mail it won’t work.
Hope you understand what I was trying to say?
Can somebody tell me what goes wrong?