when working with multiple selection checkboxes, the names for them should all be the same and end with [], for example:
<input type="checkbox" name="interet[]" value="Relation d'aide" id="interet_1">
then in the code to write the value in the email, use the implode function to create a comma separated list:
$MailBody = $MailBody . ((isset($_POST["interet"]))?implode(",", $_POST["interet"]):"");