The mail function accepts up to 5 parameters and they are entered into the mail()in this order To:, Subject:, Message:, [Additional Headers], [Additional Parameters] so your mail function should look like
$headers = 'From: youremailstuff@yourisp.com' . "\r\n" .
'Reply-To: youremailstuff@yourisp.com' . "\r\n" .
'X-Mailer: PHP/' . phpversion();
mail('emailTo.email.net','Important registration information',$message,$header);