Emailový skript hlásí SMTP server response: 550 5.7.1

Warning: mail() [function.mail]: SMTP server response: 550 5.7.1 Unable to relay for muj@email.cz in C:\xampp\htdocs\rpr\odesliemail.php on line 207

formular
<form action="odesliemail.php" method="post">
<table width="200" border="0" class="text">
<tr><td width="50"><b>J</b>méno :</td><td><input name="jmeno" accesskey="j" type="text"></td></tr>
<tr><td width="50"><b>E</b>mail :</td><td><input name="email" accesskey="e" type="text"></td></tr>
<tr><td width="50" valign="top"><b>T</b>ext :</td><td><textarea name="text" accesskey="t" rows="5" cols="25"></textarea></td></tr>
<tr><td colspan="2" align="center"><input type="submit" class="button" value="Odeslat"> &nbsp; <input type="reset" value="Vymazat"></td></tr>
</form></table>


skript vypada takto:
<?php

$to = "muj@email.cz";
$extra = "From: $email\r\nReply-To: $email\r\n";
$subject = "Vzkaz od ".$jmeno."";
$mess = "Jméno: ".$jmeno."\nEmail: ".$email."\nText:\n".$text."";

mail ($to, $subject, $mess, $extra);

?>


proc to hlasi porad chybu?
Jinak muj@email.cz se bude menit, muzou to byt ruzne emaily od ruznych lidi....
jak nastavit odesilani emailu, aby odesilani fungovalo na jakykoliv email



dekuji

muze to byt nefunkcni jen na localhostu? a na strankach to bude funkcni?