Edit: I just noted that you are trying to use a gmail address as the from value. This is not going to work, and the ISP is right in overwriting it. If you want to redirect the replies to your outgoing messages, use reply-to
.
A workaround for valid addresses that works with many ISPs:
try adding a fifth parameter to your mail()
command:
mail($to,$subject,$message,$headers,"-f [email protected]");