I have a couple other clients experiencing problems with PHP mail on FastHosts, too.
The code you provided looks to be what may be necessary to fix the problem. Where did you learn of this code? Is there a knowledgebase article stating it?
ini_set("sendmail_from", " user@yourdomain.com ");
This line could be added anywhere in config.php. The email address should be set to your ListMail "Bounce to email" from the Configuration page.
mail( "yourname@yourdomain.com", "Feedback Form
results",$_POST['message'], "From: $_POST['email']", "-fuser@yourdomain.com" );
It is not advisable to modify the ListMail code that calls the PHP mail() function. However, you -can- utilize the 5th parameter to PHP mail with a change in ListMail's config.php as follows:
$phpmailf = 1;
This ensures the 5th parameter is used and set to the ListMail "Bounce to email".
I am not sure if the first modification is needed when the second option is enabled - I believe they essentially do the same thing.
Regards