Intanto la tua select dovrebbe avere una forma tipo cosi

codice:
<select name="country">
<option value='-1' selected>Select Country</option>
<option value='0'>United Kingdom</option>
<option value='1'>United States</option>
<option value='2'>Rest of the World</option>
</select>
Dopodichè, in send.php:

codice:
$header = "From: $Name\n $Surname\nReply-To: $Name\n";

$subject = "enquiries";

switch($_POST['country'])
{
     case 0:$email_to = "UK@isp.com"; break;
     case 1:$email_to = "US@isp.com"; break;
     case 2:$email_to = "ALTROVE@isp.com"; break;
     default:die('Country non valido');
}


@mail($email_to, $subject ,$message ,$header ) ;