You could try this approach..
Your first page don't need altering.
In your sendmistakes.php file, you just need to set your headers and then insert the values from the post.
You could do with capturing their e-mail address in your form.
i.e.
PHP Code:
$toemail="youremail@address.com";
$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers .= "Content-Transfer-Encoding: 7bit\r\n";
$headers .= 'From: <'.$_POST['fromemail'].'>' . "\r\n";
$headers .= 'Reply-To: '.$_POST['fromemail']."\r\n";
$headers .= 'X-Mailers: PHP /'.phpversion() . "\r\n";
$subject = 'Posted mistakes';
$message = '
mistake:'.$_POST['mistake'].'<br/>
explanation:'.$_POST['whateverfield'].'<br/>
etc. etc.....
';
if (@mail($toemail,stripslashes($subject),stripslashes($message),stripslashes($headers)))
{
echo "Mail to:".$toemail." confirmed.</br>";
}
else
{
echo "Mail to".$toemail." failed. Please try again.</br>";
}