<?php
$sql = "SELECT email FROM users";
$res = mysql_query($sql) or die(mysql_error());
while($row = mysql_fetch_assoc($res) )
{
$area .= $row['email']. ", ";
}
?>
<?php
if(isset($_POST['submit'])){
$message = $_REQUEST['bericht'];
$to = $area;
@mail($to,"Message","$message". "\n\n". "Met vriendelijke groet,". "\n". "Brian","From: noreply@$siteurl");
Take out the explode and try with just the code above. mail() wants multiple address in the to field separated by commas, so it doesn't make any sense to explode them on commas. As andrewsi said, that is then passing to the mail function as an array.