diff --git a/src/app/Http/Controllers/API/SignupController.php b/src/app/Http/Controllers/API/SignupController.php
index 89e999a8..950d9050 100644
--- a/src/app/Http/Controllers/API/SignupController.php
+++ b/src/app/Http/Controllers/API/SignupController.php
@@ -1,174 +1,260 @@
 <?php
 
 namespace App\Http\Controllers\API;
 
 use App\Http\Controllers\Controller;
 use App\SignupCode;
 use App\User;
 
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\Auth;
 use Illuminate\Support\Facades\Validator;
 use Illuminate\Support\Str;
 
+/**
+ * Signup process API
+ */
 class SignupController extends Controller
 {
+    /** @var SignupCode A verification code object */
     protected $code;
 
+
     /**
      * Starts signup process.
      *
      * Verifies user name and email/phone, sends verification email/sms message.
      * Returns the verification code.
      *
      * @param Illuminate\Http\Request HTTP request
      *
      * @return \Illuminate\Http\JsonResponse JSON response
      */
     public function init(Request $request)
     {
-        // Validate user name and email
-        // TODO: Extended validation and support for phone number
+        // Check required fields
         $v = Validator::make(
             $request->all(),
             [
-                'email' => 'required|email',
+                'email' => 'required',
                 'name' => 'required',
             ]
         );
 
         if ($v->fails()) {
             return response()->json(['status' => 'error', 'errors' => $v->errors()], 422);
         }
 
+        // Validate user email (or phone)
+        if ($error = $this->validatePhoneOrEmail($request->email, $is_phone)) {
+            return response()->json(['status' => 'error', 'errors' => ['email' => $error]], 422);
+        }
+
         // Generate the verification code
         $code = SignupCode::create([
                 'data' => [
                     'email' => $request->email,
                     'name' => $request->name,
                 ]
         ]);
 
-        // TODO: send email/sms message
+        // Send email/sms message
+        $this->{$is_phone ? 'sendSMS' : 'sendEmail'}($code);
 
         return response()->json(['status' => 'success', 'code' => $code->code]);
     }
 
     /**
      * Validation of the verification code.
      *
      * @param Illuminate\Http\Request HTTP request
      *
      * @return \Illuminate\Http\JsonResponse JSON response
      */
     public function verify(Request $request)
     {
         // Validate the request args
         $v = Validator::make(
             $request->all(),
             [
                 'code' => 'required',
                 'short_code' => 'required',
             ]
         );
 
         if ($v->fails()) {
             return response()->json(['status' => 'error', 'errors' => $v->errors()], 422);
         }
 
-        // Validate the code
+        // Validate the verification code
         $code = SignupCode::find($request->code);
 
         if (empty($code)
             || $code->isExpired()
             || Str::upper($request->short_code) !== Str::upper($code->short_code)
         ) {
             $errors = ['short_code' => "The code is invalid or expired."];
             return response()->json(['status' => 'error', 'errors' => $errors], 422);
         }
 
         // For signup last-step mode remember the code object, so we can delete it
         // with single SQL query (->delete()) instead of two (::destroy())
         $this->code = $code;
 
         // Return user name and email/phone from the codes database on success
         return response()->json([
             'status' => 'success',
             'email' => $code->data['email'],
             'name' => $code->data['name'],
         ]);
     }
 
     /**
      * Finishes the signup process by creating the user account.
      *
      * @param Illuminate\Http\Request HTTP request
      *
      * @return \Illuminate\Http\JsonResponse JSON response
      */
     public function signup(Request $request)
     {
         // Validate input
         $v = Validator::make(
             $request->all(),
             [
                 'domain' => 'required|min:3',
                 'login' => 'required|min:2',
                 'password' => 'required|min:3|confirmed',
             ]
         );
 
         if ($v->fails()) {
             return response()->json(['status' => 'error', 'errors' => $v->errors()], 422);
         }
 
         $login = $request->login . '@' . $request->domain;
 
-        // TODO: check if specified domain is ours
-        // TODO: validate login
+        // Validate login (email)
+        if ($error = $this->validateEmail($login, true)) {
+            return response()->json(['status' => 'error', 'errors' => ['login' => $error]], 422);
+        }
 
         // Validate verification codes (again)
         $v = $this->verify($request);
         if ($v->status() !== 200) {
             return $v;
         }
 
         $code_data  = $v->getData();
         $user_name  = $code_data->name;
         $user_email = $code_data->email;
 
-        // TODO: check if user with specified login already exists
+        // We allow only ASCII, so we can safely lower-case the email address
+        $login = Str::lower($login);
 
         $user = User::create(
             [
-                // TODO: Save the external email (or phone) ?
+                // TODO: Save the external email (or phone)
                 'name' => $user_name,
                 'email' => $login,
                 'password' => $request->password,
             ]
         );
 
         // Remove the verification code
         $this->code->delete();
 
         $token = auth()->login($user);
 
-        return $this->respondWithToken($token);
+        return response()->json([
+                'access_token' => $token,
+                'token_type' => 'bearer',
+                'expires_in' => Auth::guard()->factory()->getTTL() * 60,
+        ]);
     }
 
     /**
-     * Get the token array structure.
+     * Checks if the input string is a valid email address or a phone number
      *
-     * @param string $token Respond with this token.
+     * @param string $email     Email address or phone number
+     * @param bool   &$is_phone Will be set to True if the string is valid phone number
      *
-     * @return \Illuminate\Http\JsonResponse JSON response
+     * @return string Error message on validation error
      */
-    protected function respondWithToken($token)
+    protected function validatePhoneOrEmail($input, &$is_phone = false)
     {
-        return response()->json([
-                'access_token' => $token,
-                'token_type' => 'bearer',
-                'expires_in' => Auth::guard()->factory()->getTTL() * 60,
-        ]);
+        $is_phone = false;
+
+        if (strpos($input, '@')) {
+            return $this->validateEmail($input);
+        }
+
+        $input = str_replace(array('-', ' '), '', $input);
+
+        if (!preg_match('/^\+?[0-9]{9,12}$/', $input)) {
+            return __('validation.noemailorphone');
+        }
+
+        $is_phone = true;
+    }
+
+    /**
+     * Email address validation
+     *
+     * @param string $email  Email address
+     * @param bool   $signup Enables additional checks for signup mode
+     *
+     * @return string Error message on validation error
+     */
+    protected function validateEmail($email, $signup = false)
+    {
+        $v = Validator::make(['email' => $email], ['email' => 'required|email']);
+
+        if ($v->fails()) {
+            return __('validation.emailinvalid');
+        }
+
+        // Extended checks for an address that is supposed to become a login to Kolab
+        if ($signup) {
+            list($local, $domain) = explode('@', $email);
+
+            // Local part validation
+            if (!preg_match('/^[A-Za-z0-9_.-]+$/', $local)) {
+                return __('validation.emailinvalid');
+            }
+
+            // Check if the local part is not one of exceptions
+            $exceptions = '/^(admin|administrator|sales|root)$/i';
+            if (preg_match($exceptions, $local)) {
+                return __('validation.emailexists');
+            }
+
+            // Check if user with specified login already exists
+            if (User::where('email', $email)->first()) {
+                return __('validation.emailexists');
+            }
+
+            // TODO: check if specified domain is ours
+        }
+    }
+
+    /**
+     * Sends SMS message for signup verification.
+     *
+     * @param App\SignupCode $code Verification code object
+     */
+    protected function sendSMS(SignupCode $code)
+    {
+        // TODO
+    }
+
+    /**
+     * Sends Email message for signup verification.
+     *
+     * @param App\SignupCode $code Verification code object
+     */
+    protected function sendEmail(SignupCode $code)
+    {
+        // TODO
     }
 }
diff --git a/src/resources/js/app.js b/src/resources/js/app.js
index 5b505dc5..c47e60d7 100644
--- a/src/resources/js/app.js
+++ b/src/resources/js/app.js
@@ -1,82 +1,83 @@
 /**
  * First we will load all of this project's JavaScript dependencies which
  * includes Vue and other libraries. It is a great starting point when
  * building robust, powerful web applications using Vue and Laravel.
  */
 
 require('./bootstrap')
 
 window.Vue = require('vue')
 
 import router from '../vue/js/routes.js'
 
 import AppComponent from '../vue/components/App'
 
 const app = new Vue({
     el: '#app',
     components: { AppComponent },
     router,
     methods: {
         clearFormValidation: form => {
             $(form).find('.is-invalid').removeClass('is-invalid')
             $(form).find('.invalid-feedback').remove()
         }
     },
     mounted() {
         this.$root.$on('clearFormValidation', (form) => {
             this.clearFormValidation(form)
         })
     }
 })
 
 import VueToastr from '@deveodk/vue-toastr'
 // You need a specific loader for CSS files like https://github.com/webpack/css-loader
 // If you would like custom styling of the toastr the css file can be replaced
 import '@deveodk/vue-toastr/dist/@deveodk/vue-toastr.css'
 
 Vue.use(VueToastr, {
     defaultPosition: 'toast-bottom-right',
     defaultTimeout: 50000
 })
 
 // Add a response interceptor for general/validation error handler
 window.axios.interceptors.response.use(
     response => {
         // Do nothing
         return response
     },
     error => {
         var error_msg
 
         if (error.response && error.response.status == 422) {
             error_msg = "Form validation error"
 
             $.each(error.response.data.errors || {}, (idx, msg) => {
                 $('form').each((i, form) => {
                     const input_name = ($(form).data('validation-prefix') || '') + idx
                     const input = $('#' + input_name)
 
                     if (input.length) {
                         input.addClass('is-invalid')
-                            .parent().append($('<div class="invalid-feedback">').text(msg.join('<br>')))
+                            .parent().append($('<div class="invalid-feedback">')
+                                .text($.type(msg) === 'string' ? msg : msg.join('<br>')))
 
                         return false
                     }
                 });
             })
 
             $('form .is-invalid').first().focus()
         }
         else if (error.response && error.response.data) {
             error_msg = error.response.data.message
         }
         else {
             error_msg = error.request ? error.request.statusText : error.message
         }
 
         app.$toastr('error', error_msg || "Server Error", 'Error')
 
         // Pass the error as-is
         return Promise.reject(error)
     }
 )
diff --git a/src/resources/lang/en/validation.php b/src/resources/lang/en/validation.php
index e1d879f3..57695f52 100644
--- a/src/resources/lang/en/validation.php
+++ b/src/resources/lang/en/validation.php
@@ -1,150 +1,154 @@
 <?php
 
 return [
 
     /*
     |--------------------------------------------------------------------------
     | Validation Language Lines
     |--------------------------------------------------------------------------
     |
     | The following language lines contain the default error messages used by
     | the validator class. Some of these rules have multiple versions such
     | as the size rules. Feel free to tweak each of these messages here.
     |
     */
 
     'accepted' => 'The :attribute must be accepted.',
     'active_url' => 'The :attribute is not a valid URL.',
     'after' => 'The :attribute must be a date after :date.',
     'after_or_equal' => 'The :attribute must be a date after or equal to :date.',
     'alpha' => 'The :attribute may only contain letters.',
     'alpha_dash' => 'The :attribute may only contain letters, numbers, dashes and underscores.',
     'alpha_num' => 'The :attribute may only contain letters and numbers.',
     'array' => 'The :attribute must be an array.',
     'before' => 'The :attribute must be a date before :date.',
     'before_or_equal' => 'The :attribute must be a date before or equal to :date.',
     'between' => [
         'numeric' => 'The :attribute must be between :min and :max.',
         'file' => 'The :attribute must be between :min and :max kilobytes.',
         'string' => 'The :attribute must be between :min and :max characters.',
         'array' => 'The :attribute must have between :min and :max items.',
     ],
     'boolean' => 'The :attribute field must be true or false.',
     'confirmed' => 'The :attribute confirmation does not match.',
     'date' => 'The :attribute is not a valid date.',
     'date_equals' => 'The :attribute must be a date equal to :date.',
     'date_format' => 'The :attribute does not match the format :format.',
     'different' => 'The :attribute and :other must be different.',
     'digits' => 'The :attribute must be :digits digits.',
     'digits_between' => 'The :attribute must be between :min and :max digits.',
     'dimensions' => 'The :attribute has invalid image dimensions.',
     'distinct' => 'The :attribute field has a duplicate value.',
     'email' => 'The :attribute must be a valid email address.',
     'ends_with' => 'The :attribute must end with one of the following: :values',
     'exists' => 'The selected :attribute is invalid.',
     'file' => 'The :attribute must be a file.',
     'filled' => 'The :attribute field must have a value.',
     'gt' => [
         'numeric' => 'The :attribute must be greater than :value.',
         'file' => 'The :attribute must be greater than :value kilobytes.',
         'string' => 'The :attribute must be greater than :value characters.',
         'array' => 'The :attribute must have more than :value items.',
     ],
     'gte' => [
         'numeric' => 'The :attribute must be greater than or equal :value.',
         'file' => 'The :attribute must be greater than or equal :value kilobytes.',
         'string' => 'The :attribute must be greater than or equal :value characters.',
         'array' => 'The :attribute must have :value items or more.',
     ],
     'image' => 'The :attribute must be an image.',
     'in' => 'The selected :attribute is invalid.',
     'in_array' => 'The :attribute field does not exist in :other.',
     'integer' => 'The :attribute must be an integer.',
     'ip' => 'The :attribute must be a valid IP address.',
     'ipv4' => 'The :attribute must be a valid IPv4 address.',
     'ipv6' => 'The :attribute must be a valid IPv6 address.',
     'json' => 'The :attribute must be a valid JSON string.',
     'lt' => [
         'numeric' => 'The :attribute must be less than :value.',
         'file' => 'The :attribute must be less than :value kilobytes.',
         'string' => 'The :attribute must be less than :value characters.',
         'array' => 'The :attribute must have less than :value items.',
     ],
     'lte' => [
         'numeric' => 'The :attribute must be less than or equal :value.',
         'file' => 'The :attribute must be less than or equal :value kilobytes.',
         'string' => 'The :attribute must be less than or equal :value characters.',
         'array' => 'The :attribute must not have more than :value items.',
     ],
     'max' => [
         'numeric' => 'The :attribute may not be greater than :max.',
         'file' => 'The :attribute may not be greater than :max kilobytes.',
         'string' => 'The :attribute may not be greater than :max characters.',
         'array' => 'The :attribute may not have more than :max items.',
     ],
     'mimes' => 'The :attribute must be a file of type: :values.',
     'mimetypes' => 'The :attribute must be a file of type: :values.',
     'min' => [
         'numeric' => 'The :attribute must be at least :min.',
         'file' => 'The :attribute must be at least :min kilobytes.',
         'string' => 'The :attribute must be at least :min characters.',
         'array' => 'The :attribute must have at least :min items.',
     ],
     'not_in' => 'The selected :attribute is invalid.',
     'not_regex' => 'The :attribute format is invalid.',
     'numeric' => 'The :attribute must be a number.',
     'present' => 'The :attribute field must be present.',
     'regex' => 'The :attribute format is invalid.',
     'required' => 'The :attribute field is required.',
     'required_if' => 'The :attribute field is required when :other is :value.',
     'required_unless' => 'The :attribute field is required unless :other is in :values.',
     'required_with' => 'The :attribute field is required when :values is present.',
     'required_with_all' => 'The :attribute field is required when :values are present.',
     'required_without' => 'The :attribute field is required when :values is not present.',
     'required_without_all' => 'The :attribute field is required when none of :values are present.',
     'same' => 'The :attribute and :other must match.',
     'size' => [
         'numeric' => 'The :attribute must be :size.',
         'file' => 'The :attribute must be :size kilobytes.',
         'string' => 'The :attribute must be :size characters.',
         'array' => 'The :attribute must contain :size items.',
     ],
     'starts_with' => 'The :attribute must start with one of the following: :values',
     'string' => 'The :attribute must be a string.',
     'timezone' => 'The :attribute must be a valid zone.',
     'unique' => 'The :attribute has already been taken.',
     'uploaded' => 'The :attribute failed to upload.',
     'url' => 'The :attribute format is invalid.',
     'uuid' => 'The :attribute must be a valid UUID.',
 
+    'emailexists' => 'The specified email address already exists',
+    'emailinvalid' => 'The specified email address is invalid',
+    'noemailorphone' => 'The specified text is neither a valid email address nor a phone number',
+
     /*
     |--------------------------------------------------------------------------
     | Custom Validation Language Lines
     |--------------------------------------------------------------------------
     |
     | Here you may specify custom validation messages for attributes using the
     | convention "attribute.rule" to name the lines. This makes it quick to
     | specify a specific custom language line for a given attribute rule.
     |
     */
 
     'custom' => [
         'attribute-name' => [
             'rule-name' => 'custom-message',
         ],
     ],
 
     /*
     |--------------------------------------------------------------------------
     | Custom Validation Attributes
     |--------------------------------------------------------------------------
     |
     | The following language lines are used to swap our attribute placeholder
     | with something more reader friendly such as "E-Mail Address" instead
     | of "email". This simply helps us make our message more expressive.
     |
     */
 
     'attributes' => [],
 
 ];