$(function()
{
    $("#mail_form").submit(function()
    {
        var email = $("#sender_email").val();
        if(!(/.+\@.+\..+/.test(email)))
        {
            alert("Invalid E-mail Address!");
            $("#sender_email").select();
            return false;
        }

        if($("#body").val().length == 0)
        {
            alert("You didn't write a message");
            $("#body").select();
            return false;
        }
    });
});