As placeholder in an input field? I simply use "E-mail", "Enter e-mail" or "Your registered e-mail". I don't think your suggestion is a help to the user.
In Denmark @ is called something that can be translated to "trunk a". Trunk as in the nose of an elephant, a is also pronounced as 'af' which means off. So some people find it funny to have an email starting with sutmin@... which means suckmy@. Did it make sense? It's a dirty joke.