Webfusion VPS | Sending Email From PHP

I am trying to send an email from a php script on a Webfusion VPS.

$to = "me@atmydomain.co.uk";
$subject = "Test subject";
$msg = "Test message";
$headers = "From: you@atyourdomain.co.uk
Reply-To: you@atyourdomain.co.uk";
if(mail($to, $subject, $msg, $headers)){
echo "Sent";
}
else{
echo "Error!";
}

The page displays "Sent", but I don't receive the email.

Is there a setting that I should me making in either the script or via Plesk or elsewhere to enable the emails to be sent?

I phoned Webfusion support, but they claim they only support hardware. Webfusion's help pages just say:

I'm setting up a formmail script on my Linux VPS server? what is the path to sendmail?

The path is: $mailprog = '/var/qmail/bin/sendmail -i -t';

But it doesn't explain where to put this line of code. And if it should go in the php.ini file how do I get to that?