I have some reference number that i need to send in mail but to be hidden or at least less noticeable, so if user reply on that mail that i still have that reference number.

I tried this:

<style type="text/css">
  .mail {
    font-family: RobotoRegular;
    background: rgba(255, 255, 255, 0.0);
    opacity: 0!important;
    color: transparent!important;
    font-size: 0!important;
    overflow: hidden!important;
    width: 0!important;
    height: 0!important;
  }
</style>

Also added in span directly:

<span class="mail" style="font-family:RobotoRegular;background: rgba(255,255,255,0.0);opacity:0!important;color:transparent!important;font-size:0!important;overflow:hidden!important;width:0!important;height:0!important;">#REF' + this.id + '# </span>

But without success.Font size is not changing. Any suggestion how can i achive this?