This will happen if you use form post data in the email and the server has the magic_quotes_gpc setting in the php.ini file set to ON
That setting in the php.ini fiile should be set to off. If you cannot modify the php.ini file, try adding the following code at line 1 of your form page:
<?php
if($_SERVER['REQUEST_METHOD'] == "POST") {
foreach($_POST as $k => $v) {
$_POST[$k] = stripslashes($v);
}
}
?>