What is wrong with this code(EMail sending)

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PHP

I have hmailer installed and configured.

But each time I attempt to send a mail I get this error:

( ! ) Warning: mail(): SMTP server response: 550 The address is not valid. in C:\wamp\www\TestMail.php on line 8

<?php
$from = "myusername@yahoo.com";
$to = "myusername@gmail.com";

$message = "This is a test";
$subject = "Test";

if(mail($to,$subject,$message))//This is line 8.
{

	printf("Mail sent.");
}
else
{
	print("Mail not sent");
}
?>

What could be wrong?