Fra post til php til database

Har den her kode, som gerne skulle kunne lægge en intern mail ind i databasen, men får den her error:

Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in /home/virtual/xsix.dk/public_html/online/sendmail2.php on line 45
Unknown column 'XsiX' in 'where clause'

Og det skal nok lige tages med at den person jeg prøver at skrive til hedder XsiX.

Koden:

<?php
session_start();
if(!$_SESSION[user_id] > 0) {
header("location:index.php");
exit;
}
?>
<?php

include("connect.php");

$query = mysql_query("SELECT * FROM players WHERE id=$_SESSION[user_id]");
$user_data = mysql_fetch_assoc($query) or die(mysql_error());

$query = mysql_query("SELECT * FROM mail");
$mail_data = mysql_fetch_assoc($query) or die(mysql_error());

$query = mysql_query("SELECT * FROM players WHERE accountname=".$_POST[recipient]);
$recipient2 = mysql_fetch_assoc($query) or die(mysql_error());

$new = '1';
$sender = $user_data['accountname'];
$recipient = $recipient2['id'];
$message = $_POST['message'];
$topic = $_POST['topic'];



mysql_query("INSERT INTO mail (new, sender, recipient,message,topic,time)
VALUES ('$new', '$sender', '$recipient', '$message', '$topic', 'now()')");

echo "Your mail have been send to " . $recipient ;
?>