<form id="login">
    <input type="email" class="required"/>
</form>

<form id="signup">
    <input type="email" class="required"/>
</form>

    rules: {
                            username: {
                                required: true,
                                email: true
                            },
                            password: {
                                 required: true
                            }

                        },
                        messages: {
                            username: {
                                required: "please enter an email address",
                                email: "You must use a valid email address."
                            },
                            password:{
                                required: "",
                                password: "You must use a valid password."
                            }
                        }

messages: {
  username: {
    required: "please enter an email address",
    email: function() {
      if ($("#login")) { return "Message for login page"; }
      else if ($("#signup")) { return "Message for signup page"; }
      else { return "Generic message" }
  },
  password: ...
}