No, PHP's mail() function isn't disabled in any of x10's incredibly extensive hosting packages. However, in order to help you further, I'll need to see the code in question. On occasion, different servers will interpret certain special characters different and this could be causing the problem. Additionally, some servers require that you include the headers parameter in the mail() function. But, as I said, if I had some code to look at, I could help you to a higher degree...
Ok, I know I'm not the same guy, but I'm experiencing a similar problem. My mail function looks like this:
$to = $_POST['email'];
$subject = "New visitor";
$message = "A new visitor with the ip $ip has visited your site! \n \n The total list is: $filetext1";
$from = "firstname.lastname@example.org";
$headers = "From: $from";