problem with php function mail() on fc4

[维斯 苏]

This is the first time I use mail() in php to send email,but failed,my program is
<?
$message="abcdefghijklmnopqrstuvwxyz";
$a=mail("afcn0 at yahoo.com.cn", "没有主题", $message, "From: Mailer:PHP/");
if ($a)
echo "ok";
else
echo "fail";
?> 
on fc4 it show ok,but i didn't recieve any emails,
and on win xp sp2 i also failed it tellme "Warning: mail(): SMTP server response: 550 5.7.1 Unable to relay for afcn0 at yahoo.com.cn in d:\usr\www\html\aaa1.php on line 4
fail "
tell me why,and how to use this function to send mail,btw i close all firewalls

		
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