code: public static Constraint email() {
  		return new Constraint("email", simplePayload("email")) {
  			public boolean isValid(Object actualValue) {
  				if (actualValue != null) {
  					if (!(actualValue instanceof String) || !Utils.isValidEmail((String) actualValue)) {
  						return false;
  					}
  				}
  				return true;
  			}
  		};
  	}
docs: The 'email' constraint - field must contain a valid email.
  @return constraint