<script>

        function validateform() {

            var email = $('#txt_emailID').val();
            if (email == null || email == "") {
                alert("Email Should Be Complusory");
                return false;
            }
            var atposition = email.indexOf("@");
            var dotposition = email.lastIndexOf(".");
            if (atposition < 1 || dotposition < atposition + 2 || dotposition + 2 >= email.length) {
                alert("Please enter a valid e-mail address...!!!");
                                return false;
            }
        }

    </script>

<label for="fname">Name</label>
        <input type="text" id="txt_name" name="txt_name" placeholder="Your Name.." />

        <label for="lname">EmailID</label>
        <input type="text" id="txt_emailID" name="email" placeholder="Your EmailID.." />

        <label for="lname">Subject</label>
        <input type="text" id="txt_subject" name="txt_subject" placeholder="Your Subject.." />

        <input type="submit" id="submit_mail" value="Send" />

        <script>
            $(function () {
                $(document).on("click", "#submit_mail", function () {
                    if (validateform() == false) {
                        document.getElementById("txt_name").value = $('#txt_name').val();
                        document.getElementById("txt_subject").value = $('#txt_subject').val();

                    }
                    else {

                    }
                });
            });