Simple and lightweight jQuery email validation.

email_validation

<!-- Add the styles below to your CSS file -->
<style>
  input.email-error {
    color: #C24C4C;
  }
</style>

<!-- Add the jQuery below to your Javacript file -->
<script>
  $(document).ready( function(){
    
    // Change 'button' to 'input' if needed
    $('input[type=email]').on("keyup", function() {
      var emailTest = /^([a-zA-Z0-9_.+-])+\@(([a-zA-Z0-9-])+\.)+([a-zA-Z0-9]{2,4})+$/;
      if (!emailTest.test(this.value)) {
        $(this).addClass("email-error");
        $("button[type=submit]").attr('disabled','disabled');
      } else {
        $(this).removeClass("email-error");
        $("button[type=submit]").removeAttr('disabled');
      }
    });

  });
</script>