Its clustered linux servers i think
Check first with your hosting company support and see if they have mail() set up properly. If they either you have a problem with your code or your mail may be blocked or added to spam.
You'll need to place some debugging into your code.
for starters:
[PHP]if (!mail(....)) {
echo 'Mail is not working... ';
}[/PHP]
mail() will return Boolean true or false if working or not respectively.
Set your error reporting to max:
[PHP]error_reporting(E_ALL);
ini_set('display_errors', 'true'); // not sure bout this one, I think thats the correct config option[/PHP]
Just try a simple mail() first.. don't use the abstraction function "send_mail".. just to test if its working.
Check if your hosting company's mail servers are not blacklisted.
Send emails to a few different addresses to make sure its not specific to an email address...