Vi ringrazio, ma mi funziona solo in parte e non capisco perchè.
Lo so che sto chiedendo molto, ma vi posto il codice....

function admin_send_msg_confirm() {
// Confirms the message before sending.
global $username, $password, $subject, $message, $mess, $owner_email, $list_name, $list_file, $admin_keyword, $use_sig;
$subject = str_replace("%%", "\"", $subject);
$subject = stripslashes($subject);
$message = str_replace("%%", "\"", $message);
$message = stripslashes($message);
$mess = str_replace("%%", "\"", $mess);
$mess = stripslashes($mess);
echo "
<form method=\"post\">
<table align=\"center\" border=\"0\" width='700'>
<tr><td colspan=\"2\" align=\"center\" valign=\"top\"><font size=\"+1\">Per favore conferma che questo è ciò che vuoi spedire:
</td></tr>
<tr><td align=\"left\" valign=\"top\">From: </td><td align=\"left\" valign=\"top\">\"$list_name\" &lt;$owner_email&gt;</td></tr>
<tr><td align=\"left\" valign=\"top\">Subject: </td><td align=\"left\" valign=\"top\">$subject</td></tr>
<tr><td align=\"left\" valign=\"top\" >Message: </td><td align=\"left\" valign=\"top\" width='700'>$message</td></tr>";


$headers = "From: \"$list_name\" <$owner_email>\r\nReply-To: $owner_email\nMIME-version: 1.0\nContent-Type: text/html; charset=iso-8859-1\n boundary=\"Message-Boundary\"\nContent-Transfer-Encoding: 8BIT\n Content-description: Mail message body\n\n";
$msg_body = $mess;

mail("mia@mail.it.it", $subject, $msg_body, $headers);


echo"
<input type=\"hidden\" name=\"username\" value=\"$username\">
<input type=\"hidden\" name=\"password\" value=\"$password\">
<input type=\"hidden\" name=\"message\" value=\"$message\">
<input type=\"hidden\" name=\"subject\" value=\"$subject\">
<input type=\"hidden\" name=\"use_sig\" value=\"$use_sig\">
<input type=\"hidden\" name=\"action\" value=\"$admin_keyword\">
<input type=\"hidden\" name=\"q\" value=\"send_msg\">
<tr><td></td><td align=\"left\" valign=\"top\"><input type=\"submit\" value=\"Invia\"><input type=\"reset\" value=\"Edit\" OnClick=\"history.go(-1); return true;\"></td></tr></table>
</form>";
}

function admin_send_msg() {
// Sends the message to all subscribers on the list.
global $username, $password, $subject, $message, $mess, $owner_email, $list_name, $list_file, $admin_keyword, $use_sig;
$subject = stripslashes($GLOBALS[subject]);
$message = stripslashes($GLOBALS[message]);
$subject = str_replace("%%", "\"", $subject);
$message = str_replace("%%", "\"", $message);
$message = stripslashes($message);
$mess = str_replace("%%", "\"", $mess);
$mess = stripslashes($mess);

$headers = "From: \"$list_name\" <$owner_email>\r\nReply-To: $owner_email\nMIME-version: 1.0\nContent-Type: text/html; charset=iso-8859-1\n boundary=\"Message-Boundary\"\nContent-Transfer-Encoding: 8BIT\n Content-description: Mail message body\n\n";

$sucess_count = 0;
$fail_count = 0;

$msg_body = $mess;

$addresses = @file($list_file) or die("<center>La list data file non può essere aperta.
Check the path and permissions.</center>");
$addresses[] = $owner_email;
foreach ($addresses as $email) {
$email = trim($email);
if (mail($email, $subject, $msg_body, $headers)) $sucess_count++;
else $fail_count++;
}
echo "<h3 align=\"center\">Il tuo messaggio è stato spedito correttamente a $sucess_count iscritti</h3><h4 align=\"center\">$fail_count Non spedite.</h4>";
}

La prima funzione spedisce la mail solo a me per un controllo e con le dritte che mi avete dato funziona tutto a meraviglia. La seconda spedisce la mail a tutti gli indirizzi e continua a darmi problemi.