angular.module('auth')
    .factory('ValidationService', ['$http', '$q', function($http, $q){
        var vs = {};
        vs.uniqueEmail = function(mail) {
            return $http.post('/api/user/validate/email/', {email: mail})
                .then(
                    function(response) {
                        if (!response.data.isValid) {
                            alert(response.data.error);
                            $q.reject(response.data.error);
                        }
                        return true;
                    },
                    function(response) {
                        return $q.reject("OOPS");
                    }
                );
        };
        return vs;
    }]);

angular.module('auth')
    .directive('uniqueEmail', function(ValidationService) {
        return {
            require: 'ngModel',
            link: function(scope, elm, attrs, ctrl) {
                ctrl.$asyncValidators.uniqueEmail = function(modelValue, viewValue) {
                    var value = modelValue || viewValue;
                    return ValidationService.uniqueEmail(value);
                };
            }
        }
    });


<form novalidate class="css-form" name="usrform">
    Email: <input type="email" ng-model="user.email" name="email" unique-email required/><br/>
    <span ng-show="usrform.email.$touched && usrform.email.$error.uniqueEmail">addres is already in use</span>
</form>
<pre>email = "{{usrform.email.$valid}}"</pre>