diff --git a/assets/js/src/ajax.js b/assets/js/src/ajax.js
index eccef1c790..e76f3ad76d 100644
--- a/assets/js/src/ajax.js
+++ b/assets/js/src/ajax.js
@@ -27,11 +27,6 @@ define('ajax', ['mailpoet', 'jquery', 'underscore'], function(MailPoet, jQuery,
         if(this.options.token === null) {
           this.options.token = window.mailpoet_token;
         }
-
-        // set default API version
-        if(this.options.api_version === null) {
-          this.options.api_version = window.mailpoet_api_version;
-        }
       },
       getParams: function() {
         return {
diff --git a/assets/js/src/form/form.jsx b/assets/js/src/form/form.jsx
index ca61b30d8c..bfffe4eb86 100644
--- a/assets/js/src/form/form.jsx
+++ b/assets/js/src/form/form.jsx
@@ -64,6 +64,7 @@ define(
         this.setState({ loading: true });
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: this.props.endpoint,
           action: 'get',
           data: {
@@ -112,6 +113,7 @@ define(
         }
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: this.props.endpoint,
           action: 'save',
           data: item
diff --git a/assets/js/src/forms/list.jsx b/assets/js/src/forms/list.jsx
index 43d3c7f261..979374c58e 100644
--- a/assets/js/src/forms/list.jsx
+++ b/assets/js/src/forms/list.jsx
@@ -97,6 +97,7 @@ const item_actions = [
     label: MailPoet.I18n.t('duplicate'),
     onClick: function(item, refresh) {
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'forms',
         action: 'duplicate',
         data: {
@@ -125,6 +126,7 @@ const item_actions = [
 const FormList = React.createClass({
   createForm() {
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'forms',
       action: 'create'
     }).done((response) => {
diff --git a/assets/js/src/listing/listing.jsx b/assets/js/src/listing/listing.jsx
index 0af6825e7f..2c28bf5cbb 100644
--- a/assets/js/src/listing/listing.jsx
+++ b/assets/js/src/listing/listing.jsx
@@ -445,6 +445,7 @@ const Listing = React.createClass({
       this.clearSelection();
 
       MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: this.props.endpoint,
         action: 'listing',
         data: {
@@ -495,6 +496,7 @@ const Listing = React.createClass({
     });
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: this.props.endpoint,
       action: 'restore',
       data: {
@@ -522,6 +524,7 @@ const Listing = React.createClass({
     });
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: this.props.endpoint,
       action: 'trash',
       data: {
@@ -549,6 +552,7 @@ const Listing = React.createClass({
     });
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: this.props.endpoint,
       action: 'delete',
       data: {
@@ -611,6 +615,7 @@ const Listing = React.createClass({
     }
 
     return MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: this.props.endpoint,
       action: 'bulkAction',
       data: data
diff --git a/assets/js/src/newsletter_editor/components/communication.js b/assets/js/src/newsletter_editor/components/communication.js
index 56368103dc..2b765b8144 100644
--- a/assets/js/src/newsletter_editor/components/communication.js
+++ b/assets/js/src/newsletter_editor/components/communication.js
@@ -9,6 +9,7 @@ define([
 
   Module._query = function(args) {
     return MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'automatedLatestContent',
       action: args.action,
       data: args.options || {}
@@ -81,6 +82,7 @@ define([
 
   Module.saveNewsletter = function(options) {
     return MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'save',
       data: options || {}
@@ -89,6 +91,7 @@ define([
 
   Module.previewNewsletter = function(options) {
     return MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'sendPreview',
       data: options || {}
diff --git a/assets/js/src/newsletter_editor/components/save.js b/assets/js/src/newsletter_editor/components/save.js
index e09d592455..2c45fa0ba2 100644
--- a/assets/js/src/newsletter_editor/components/save.js
+++ b/assets/js/src/newsletter_editor/components/save.js
@@ -109,6 +109,7 @@ define([
       });
 
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'newsletterTemplates',
         action: 'save',
         data: data
diff --git a/assets/js/src/newsletter_editor/components/sidebar.js b/assets/js/src/newsletter_editor/components/sidebar.js
index 4419b7bf05..cbc57def62 100644
--- a/assets/js/src/newsletter_editor/components/sidebar.js
+++ b/assets/js/src/newsletter_editor/components/sidebar.js
@@ -243,6 +243,7 @@ define([
       MailPoet.Modal.loading(true);
 
       MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'newsletters',
         action: 'showPreview',
         data: json,
diff --git a/assets/js/src/newsletters/listings/mixins.jsx b/assets/js/src/newsletters/listings/mixins.jsx
index 3973ff9e65..98555a8815 100644
--- a/assets/js/src/newsletters/listings/mixins.jsx
+++ b/assets/js/src/newsletters/listings/mixins.jsx
@@ -8,6 +8,7 @@ import jQuery from 'jquery'
 const _QueueMixin = {
   pauseSending: function(newsletter) {
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'sendingQueue',
       action: 'pause',
       data: {
@@ -27,6 +28,7 @@ const _QueueMixin = {
   },
   resumeSending: function(newsletter) {
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'sendingQueue',
       action: 'resume',
       data: {
@@ -225,6 +227,7 @@ const _MailerMixin = {
   },
   resumeMailerSending() {
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'mailer',
       action: 'resumeSending'
     }).done(function() {
diff --git a/assets/js/src/newsletters/listings/notification.jsx b/assets/js/src/newsletters/listings/notification.jsx
index 1f59fc36d4..256bcf4046 100644
--- a/assets/js/src/newsletters/listings/notification.jsx
+++ b/assets/js/src/newsletters/listings/notification.jsx
@@ -129,6 +129,7 @@ const newsletter_actions = [
     label: MailPoet.I18n.t('duplicate'),
     onClick: function(newsletter, refresh) {
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'newsletters',
         action: 'duplicate',
         data: {
@@ -164,6 +165,7 @@ const NewsletterListNotification = React.createClass({
     e.persist();
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'setStatus',
       data: {
diff --git a/assets/js/src/newsletters/listings/standard.jsx b/assets/js/src/newsletters/listings/standard.jsx
index 1c56229585..f399df617e 100644
--- a/assets/js/src/newsletters/listings/standard.jsx
+++ b/assets/js/src/newsletters/listings/standard.jsx
@@ -124,6 +124,7 @@ const newsletter_actions = [
     label: MailPoet.I18n.t('duplicate'),
     onClick: function(newsletter, refresh) {
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'newsletters',
         action: 'duplicate',
         data: {
diff --git a/assets/js/src/newsletters/listings/welcome.jsx b/assets/js/src/newsletters/listings/welcome.jsx
index bae199461f..ac4d269089 100644
--- a/assets/js/src/newsletters/listings/welcome.jsx
+++ b/assets/js/src/newsletters/listings/welcome.jsx
@@ -126,6 +126,7 @@ const newsletter_actions = [
     label: MailPoet.I18n.t('duplicate'),
     onClick: function(newsletter, refresh) {
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'newsletters',
         action: 'duplicate',
         data: {
@@ -161,6 +162,7 @@ const NewsletterListWelcome = React.createClass({
     e.persist();
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'setStatus',
       data: {
diff --git a/assets/js/src/newsletters/send.jsx b/assets/js/src/newsletters/send.jsx
index 1874b12513..7ccc781b94 100644
--- a/assets/js/src/newsletters/send.jsx
+++ b/assets/js/src/newsletters/send.jsx
@@ -64,6 +64,7 @@ define(
         this.setState({ loading: true });
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'get',
           data: {
@@ -97,6 +98,7 @@ define(
               case 'notification':
               case 'welcome':
                 return MailPoet.Ajax.post({
+                  api_version: window.mailpoet_api_version,
                   endpoint: 'newsletters',
                   action: 'setStatus',
                   data: {
@@ -120,6 +122,7 @@ define(
                 }).fail(this._showError);
               default:
                 return MailPoet.Ajax.post({
+                  api_version: window.mailpoet_api_version,
                   endpoint: 'sendingQueue',
                   action: 'add',
                   data: {
@@ -184,6 +187,7 @@ define(
         );
 
         return MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'save',
           data: newsletterData,
diff --git a/assets/js/src/newsletters/send/notification.jsx b/assets/js/src/newsletters/send/notification.jsx
index 539616778c..d66f637f4a 100644
--- a/assets/js/src/newsletters/send/notification.jsx
+++ b/assets/js/src/newsletters/send/notification.jsx
@@ -37,8 +37,9 @@ define(
         tip: MailPoet.I18n.t('segmentsTip'),
         type: 'selection',
         placeholder: MailPoet.I18n.t('selectSegmentPlaceholder'),
-        id: "mailpoet_segments",
-        endpoint: "segments",
+        id: 'mailpoet_segments',
+        api_version: window.mailpoet_api_version,
+        endpoint: 'segments',
         multiple: true,
         filter: function(segment) {
           return !!(!segment.deleted_at);
diff --git a/assets/js/src/newsletters/send/standard.jsx b/assets/js/src/newsletters/send/standard.jsx
index fcf5ece48b..a8187521b8 100644
--- a/assets/js/src/newsletters/send/standard.jsx
+++ b/assets/js/src/newsletters/send/standard.jsx
@@ -341,8 +341,9 @@ define(
         tip: MailPoet.I18n.t('segmentsTip'),
         type: 'selection',
         placeholder: MailPoet.I18n.t('selectSegmentPlaceholder'),
-        id: "mailpoet_segments",
-        endpoint: "segments",
+        id: 'mailpoet_segments',
+        api_version: window.mailpoet_api_version,
+        endpoint: 'segments',
         multiple: true,
         filter: function(segment) {
           return !!(!segment.deleted_at);
diff --git a/assets/js/src/newsletters/templates.jsx b/assets/js/src/newsletters/templates.jsx
index 5ef22ff2a6..f292c5a0ff 100644
--- a/assets/js/src/newsletters/templates.jsx
+++ b/assets/js/src/newsletters/templates.jsx
@@ -27,6 +27,7 @@ define(
         MailPoet.Modal.loading(true);
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletterTemplates',
           action: 'save',
           data: template
@@ -97,6 +98,7 @@ define(
         MailPoet.Modal.loading(true);
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletterTemplates',
           action: 'getAll',
         }).always(() => {
@@ -130,6 +132,7 @@ define(
         }
 
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'save',
           data: {
@@ -158,6 +161,7 @@ define(
           )
         ) {
           MailPoet.Ajax.post({
+            api_version: window.mailpoet_api_version,
             endpoint: 'newsletterTemplates',
             action: 'delete',
             data: {
diff --git a/assets/js/src/newsletters/types.jsx b/assets/js/src/newsletters/types.jsx
index 65cadff270..dca520ac69 100644
--- a/assets/js/src/newsletters/types.jsx
+++ b/assets/js/src/newsletters/types.jsx
@@ -22,6 +22,7 @@ define(
       },
       createNewsletter: function(type) {
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'create',
           data: {
diff --git a/assets/js/src/newsletters/types/notification/notification.jsx b/assets/js/src/newsletters/types/notification/notification.jsx
index 591eea0955..10210b7a02 100644
--- a/assets/js/src/newsletters/types/notification/notification.jsx
+++ b/assets/js/src/newsletters/types/notification/notification.jsx
@@ -44,6 +44,7 @@ define(
       },
       handleNext: function() {
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'create',
           data: _.extend({}, this.state, {
diff --git a/assets/js/src/newsletters/types/standard.jsx b/assets/js/src/newsletters/types/standard.jsx
index 5762902826..9d776a0c66 100644
--- a/assets/js/src/newsletters/types/standard.jsx
+++ b/assets/js/src/newsletters/types/standard.jsx
@@ -22,6 +22,7 @@ define(
       componentDidMount: function() {
         // No options for this type, create a newsletter upon mounting
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'create',
           data: {
diff --git a/assets/js/src/newsletters/types/welcome/scheduling.jsx b/assets/js/src/newsletters/types/welcome/scheduling.jsx
index c7e1fa2418..d43593514d 100644
--- a/assets/js/src/newsletters/types/welcome/scheduling.jsx
+++ b/assets/js/src/newsletters/types/welcome/scheduling.jsx
@@ -104,6 +104,7 @@ const WelcomeScheduling = React.createClass({
   },
   handleNext: function() {
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'create',
       data: {
diff --git a/assets/js/src/newsletters/types/welcome/welcome.jsx b/assets/js/src/newsletters/types/welcome/welcome.jsx
index 18bb917065..78facf4009 100644
--- a/assets/js/src/newsletters/types/welcome/welcome.jsx
+++ b/assets/js/src/newsletters/types/welcome/welcome.jsx
@@ -52,6 +52,7 @@ define(
       },
       handleNext: function() {
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'newsletters',
           action: 'create',
           data: _.extend({}, this.state, {
diff --git a/assets/js/src/public.js b/assets/js/src/public.js
index 186025c280..4b3d570603 100644
--- a/assets/js/src/public.js
+++ b/assets/js/src/public.js
@@ -40,8 +40,8 @@ function(
             // ajax request
             MailPoet.Ajax.post({
               url: MailPoetForm.ajax_url,
-              api_version: form_data.api_version,
               token: form_data.token,
+              api_version: form_data.api_version,
               endpoint: 'subscribers',
               action: 'subscribe',
               data: form_data.data
diff --git a/assets/js/src/segments/list.jsx b/assets/js/src/segments/list.jsx
index cfc15bc4f1..f27ebfacef 100644
--- a/assets/js/src/segments/list.jsx
+++ b/assets/js/src/segments/list.jsx
@@ -112,6 +112,7 @@ const item_actions = [
     label: MailPoet.I18n.t('duplicate'),
     onClick: (item, refresh) => {
       return MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'segments',
         action: 'duplicate',
         data: {
@@ -153,6 +154,7 @@ const item_actions = [
     onClick: function(item, refresh) {
       MailPoet.Modal.loading(true);
       MailPoet.Ajax.post({
+        api_version: window.mailpoet_api_version,
         endpoint: 'segments',
         action: 'synchronize'
       }).done(function(response) {
diff --git a/assets/js/src/subscribers/form.jsx b/assets/js/src/subscribers/form.jsx
index cdccd7e310..7e5964b798 100644
--- a/assets/js/src/subscribers/form.jsx
+++ b/assets/js/src/subscribers/form.jsx
@@ -60,7 +60,8 @@ define(
         label: MailPoet.I18n.t('lists'),
         type: 'selection',
         placeholder: MailPoet.I18n.t('selectList'),
-        endpoint: "segments",
+        api_version: window.mailpoet_api_version,
+        endpoint: 'segments',
         multiple: true,
         selected: function(subscriber) {
           if (Array.isArray(subscriber.subscriptions) === false) {
diff --git a/assets/js/src/subscribers/importExport/export.js b/assets/js/src/subscribers/importExport/export.js
index 07989cec42..f17c78273a 100644
--- a/assets/js/src/subscribers/importExport/export.js
+++ b/assets/js/src/subscribers/importExport/export.js
@@ -138,6 +138,7 @@ define(
        }
        MailPoet.Modal.loading(true);
        MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'ImportExport',
           action: 'processExport',
           data: JSON.stringify({
diff --git a/assets/js/src/subscribers/importExport/import.js b/assets/js/src/subscribers/importExport/import.js
index d2ae4da01d..e263ed90be 100644
--- a/assets/js/src/subscribers/importExport/import.js
+++ b/assets/js/src/subscribers/importExport/import.js
@@ -190,6 +190,7 @@ define(
           mailChimpKeyVerifyButtonElement.click(function () {
             MailPoet.Modal.loading(true);
             MailPoet.Ajax.post({
+              api_version: window.mailpoet_api_version,
               endpoint: 'importExport',
               action: 'getMailChimpLists',
               data: {
@@ -225,6 +226,7 @@ define(
             }
             MailPoet.Modal.loading(true);
             MailPoet.Ajax.post({
+              api_version: window.mailpoet_api_version,
               endpoint: 'importExport',
               action: 'getMailChimpSubscribers',
               data: {
@@ -572,6 +574,7 @@ define(
               var segmentDescription = jQuery('#new_segment_description').val().trim();
 
               MailPoet.Ajax.post({
+                api_version: window.mailpoet_api_version,
                 endpoint: 'ImportExport',
                 action: 'addSegment',
                 data: {
@@ -729,6 +732,7 @@ define(
 
                     // save custom field
                     MailPoet.Ajax.post({
+                      api_version: window.mailpoet_api_version,
                       endpoint: 'customFields',
                       action: 'save',
                       data: data
@@ -990,6 +994,7 @@ define(
               queue.add(function(queue) {
                 queue.pause();
                 MailPoet.Ajax.post({
+                  api_version: window.mailpoet_api_version,
                   endpoint: 'ImportExport',
                   action: 'processImport',
                   data: JSON.stringify({
diff --git a/assets/js/src/subscribers/list.jsx b/assets/js/src/subscribers/list.jsx
index 7f3d1dd279..cb263a5501 100644
--- a/assets/js/src/subscribers/list.jsx
+++ b/assets/js/src/subscribers/list.jsx
@@ -91,6 +91,7 @@ const bulk_actions = [
     onSelect: function() {
       let field = {
         id: 'move_to_segment',
+        api_version: window.mailpoet_api_version,
         endpoint: 'segments',
         filter: function(segment) {
           return !!(
@@ -122,6 +123,7 @@ const bulk_actions = [
     onSelect: function() {
       let field = {
         id: 'add_to_segment',
+        api_version: window.mailpoet_api_version,
         endpoint: 'segments',
         filter: function(segment) {
           return !!(
@@ -153,6 +155,7 @@ const bulk_actions = [
     onSelect: function() {
       let field = {
         id: 'remove_from_segment',
+        api_version: window.mailpoet_api_version,
         endpoint: 'segments',
         filter: function(segment) {
           return !!(
@@ -255,7 +258,7 @@ const SubscriberList = React.createClass({
       case 'unsubscribed':
         status = MailPoet.I18n.t('unsubscribed');
       break;
-      
+
       case 'bounced':
         status = MailPoet.I18n.t('bounced');
       break;
diff --git a/lib/API/API.php b/lib/API/API.php
index f70f0f257e..e1ec585bcb 100644
--- a/lib/API/API.php
+++ b/lib/API/API.php
@@ -1,6 +1,7 @@
 <?php
 namespace MailPoet\API;
 
+use MailPoet\Config\Env;
 use MailPoet\Util\Helpers;
 use MailPoet\Util\Security;
 use MailPoet\WP\Hooks;
@@ -8,19 +9,29 @@ use MailPoet\WP\Hooks;
 if(!defined('ABSPATH')) exit;
 
 class API {
-  private $_api_version;
-  private $_endpoint;
-  private $_method;
-  private $_token;
-
+  private $_request_api_version;
+  private $_request_endpoint;
+  private $_request_method;
+  private $_request_token;
+  private $_request_endpoint_class;
+  private $_request_data = array();
   private $_endpoint_namespaces = array();
-  private $_endpoint_class;
-  private $_data = array();
-
-  const CURRENT_VERSION = 1;
+  private $_ajax_request = false;
+  private $_available_api_versions = array(
+      'v1'
+  );
+  const CURRENT_VERSION = 'v1';
+  const ENDPOINTS_LOCATION = 'Endpoints';
 
   function __construct() {
-    $this->addEndpointNamespace(__NAMESPACE__ . "\\Endpoints");
+    foreach($this->_available_api_versions as $version) {
+      $this->addEndpointNamespace(
+        array(
+          'name' => sprintf('%s\%s', __NAMESPACE__, self::ENDPOINTS_LOCATION),
+          'version' => $version
+        )
+      );
+    }
   }
 
   function init() {
@@ -44,19 +55,14 @@ class API {
   }
 
   function setupAjax() {
+    $this->_ajax_request = true;
     Hooks::doAction('mailpoet_api_setup', array($this));
-
     $this->getRequestData($_POST);
 
     if($this->checkToken() === false) {
-      $error_response = new ErrorResponse(
-        array(
-          Error::UNAUTHORIZED => __('Invalid request', 'mailpoet')
-        ),
-        array(),
-        Response::STATUS_UNAUTHORIZED
-      );
-      $error_response->send();
+      $error_message = __('Invalid API request.', 'mailpoet');
+      $error_response = $this->createErrorResponse(Error::UNAUTHORIZED, $error_message, Response::STATUS_UNAUTHORIZED);
+      return $error_response;
     }
 
     $response = $this->processRoute();
@@ -64,61 +70,56 @@ class API {
   }
 
   function getRequestData($data) {
-    $this->_api_version = sprintf(
-      'v%s',
-      isset($data['api_version'])
-        ? $data['api_version']
-        : self::CURRENT_VERSION
-    );
+    $this->_request_api_version = !empty($data['api_version']) ? $data['api_version']: false;
 
-    $this->_endpoint = isset($data['endpoint'])
+    $this->_request_endpoint = isset($data['endpoint'])
       ? Helpers::underscoreToCamelCase(trim($data['endpoint']))
       : null;
 
     // JS part of /wp-admin/customize.php does not like a 'method' field in a form widget
     $method_param_name = isset($data['mailpoet_method']) ? 'mailpoet_method' : 'method';
-    $this->_method = isset($data[$method_param_name])
+    $this->_request_method = isset($data[$method_param_name])
       ? Helpers::underscoreToCamelCase(trim($data[$method_param_name]))
       : null;
 
-    $this->_token = isset($data['token'])
+    $this->_request_token = isset($data['token'])
       ? trim($data['token'])
       : null;
 
-    if(!$this->_endpoint || !$this->_method) {
-      // throw exception bad request
-      $error_response = new ErrorResponse(
-        array(
-          Error::BAD_REQUEST => __('Invalid request', 'mailpoet')
-        ),
-        array(),
-        Response::STATUS_BAD_REQUEST
-      );
-      $error_response->send();
+    if(!$this->_request_endpoint || !$this->_request_method || !$this->_request_api_version) {
+      $error_message = __('Invalid API request.', 'mailpoet');
+      $error_response = $this->createErrorResponse(Error::BAD_REQUEST, $error_message, Response::STATUS_BAD_REQUEST);
+      return $error_response;
     } else {
       foreach($this->_endpoint_namespaces as $namespace) {
-        $class_name = $namespace . "\\" . $this->_api_version . "\\" . ucfirst($this->_endpoint);
-        if(class_exists($class_name)) {
-          $this->_endpoint_class = $class_name;
+        if($namespace['version'] !== $this->_request_api_version) continue;
+        $endpoint_class = sprintf(
+          '%s\%s\%s',
+          $namespace['name'],
+          $namespace['version'],
+          ucfirst($this->_request_endpoint)
+        );
+        if(class_exists($endpoint_class)) {
+          $this->_request_endpoint_class = $endpoint_class;
         }
       }
-
-      $this->_data = isset($data['data'])
+      $this->_request_data = isset($data['data'])
         ? stripslashes_deep($data['data'])
         : array();
 
       // remove reserved keywords from data
-      if(is_array($this->_data) && !empty($this->_data)) {
+      if(is_array($this->_request_data) && !empty($this->_request_data)) {
         // filter out reserved keywords from data
         $reserved_keywords = array(
           'token',
           'endpoint',
           'method',
+          'api_version',
           'mailpoet_method', // alias of 'method'
           'mailpoet_redirect'
         );
-        $this->_data = array_diff_key(
-          $this->_data,
+        $this->_request_data = array_diff_key(
+          $this->_request_data,
           array_flip($reserved_keywords)
         );
       }
@@ -127,57 +128,46 @@ class API {
 
   function processRoute() {
     try {
-      if(empty($this->_endpoint_class)) {
-        throw new \Exception('Invalid endpoint');
+      if(empty($this->_request_endpoint_class)) {
+        throw new \Exception(__('Invalid API endpoint.', 'mailpoet'));
       }
 
-      $endpoint = new $this->_endpoint_class();
+      $endpoint = new $this->_request_endpoint_class();
 
       // check the accessibility of the requested endpoint's action
       // by default, an endpoint's action is considered "private"
       $permissions = $endpoint->permissions;
-      if(
-        array_key_exists($this->_method, $permissions) === false
-        ||
-        $permissions[$this->_method] !== Access::ALL
+      if(array_key_exists($this->_request_method, $permissions) === false ||
+         $permissions[$this->_request_method] !== Access::ALL
       ) {
         if($this->checkPermissions() === false) {
-          $error_response = new ErrorResponse(
-            array(
-              Error::FORBIDDEN => __(
-                'You do not have the required permissions.',
-                'mailpoet'
-              )
-            ),
-            array(),
-            Response::STATUS_FORBIDDEN
-          );
+          $error_message = __('You do not have the required permissions.', 'mailpoet');
+          $error_response = $this->createErrorResponse(Error::FORBIDDEN, $error_message, Response::STATUS_FORBIDDEN);
           return $error_response;
         }
       }
 
-      $response = $endpoint->{$this->_method}($this->_data);
+      $response = $endpoint->{$this->_request_method}($this->_request_data);
       return $response;
     } catch(\Exception $e) {
-      $error_response = new ErrorResponse(
-        array($e->getCode() => $e->getMessage())
-      );
+      $error_message = $e->getMessage();
+      $error_response = $this->createErrorResponse(Error::BAD_REQUEST, $error_message, Response::STATUS_BAD_REQUEST);
       return $error_response;
     }
   }
 
   function checkPermissions() {
-    return current_user_can('manage_options');
+    return current_user_can(Env::$required_permission);
   }
 
   function checkToken() {
-    return wp_verify_nonce($this->_token, 'mailpoet_token');
+    return wp_verify_nonce($this->_request_token, 'mailpoet_token');
   }
 
   function setTokenAndAPIVersion() {
     $global = '<script type="text/javascript">';
     $global .= 'var mailpoet_token = "%s";';
-    $global .= 'var mailpoet_api_version = %s;';
+    $global .= 'var mailpoet_api_version = "%s";';
     $global .= '</script>';
     echo sprintf(
       $global,
@@ -186,7 +176,10 @@ class API {
     );
   }
 
-  function addEndpointNamespace($namespace) {
+  function addEndpointNamespace(array $namespace) {
+    if(empty($namespace['version'])) {
+      throw new \Exception(__('Namespace version is required.', 'mailpoet'));
+    }
     $this->_endpoint_namespaces[] = $namespace;
   }
 
@@ -195,10 +188,21 @@ class API {
   }
 
   function getRequestedEndpointClass() {
-    return $this->_endpoint_class;
+    return $this->_request_endpoint_class;
   }
 
   function getRequestedAPIVersion() {
-    return $this->_api_version;
+    return $this->_request_api_version;
+  }
+
+  function createErrorResponse($error_type, $error_message, $response_status) {
+    $error_response = new ErrorResponse(
+      array(
+        $error_type => $error_message
+      ),
+      array(),
+      $response_status
+    );
+    return ($this->_ajax_request === true) ? $error_response->send() : $error_response;
   }
 }
\ No newline at end of file
diff --git a/tests/unit/API/APITest.php b/tests/unit/API/APITest.php
index 2a1ebbf823..ab0348a70c 100644
--- a/tests/unit/API/APITest.php
+++ b/tests/unit/API/APITest.php
@@ -1,7 +1,9 @@
 <?php
+
 use Codeception\Util\Stub;
 use MailPoet\API\API;
 use MailPoet\API\SuccessResponse;
+use MailPoet\API\Response as APIResponse;
 
 // required to be able to use wp_delete_user()
 require_once(ABSPATH.'wp-admin/includes/user.php');
@@ -59,7 +61,10 @@ class APITest extends MailPoetTest {
   function testItCanAddEndpointNamespaces() {
     expect($this->api->getEndpointNamespaces())->count(1);
 
-    $namespace = "MailPoet\\Dummy\\Name\\Space";
+    $namespace = array(
+      'name' => 'MailPoet\\Dummy\\Name\\Space',
+      'version' => 'v2'
+    );
     $this->api->addEndpointNamespace($namespace);
     $namespaces = $this->api->getEndpointNamespaces();
 
@@ -67,30 +72,39 @@ class APITest extends MailPoetTest {
     expect($namespaces[1])->equals($namespace);
   }
 
-  function testItDefaultsToCurrentAPIVersionWhenNoVersionIsSpecified() {
-    $namespace = "MailPoet\\Some\\Name\\Space\\Endpoints";
-    $this->api->addEndpointNamespace($namespace);
+  function testItRequiresNamespaceVersionToBeSpecified() {
+    $namespace = array(
+      'name' => 'MailPoet\\Dummy\\Name\\Space'
+    );
 
+    try {
+      $this->api->addEndpointNamespace($namespace);
+      $this->fail('Exception was not thrown');
+    } catch(Exception $e) {
+      expect($e->getMessage())->equals('Namespace version is required.');
+    }
+  }
+
+  function testItReturns400ErrorWhenAPIVersionIsNotSpecified() {
     $data = array(
       'endpoint' => 'namespaced_endpoint_stub',
       'method' => 'test'
     );
-    $this->api->getRequestData($data);
-    expect($this->api->getRequestedEndpointClass())->equals(
-      sprintf(
-        'MailPoet\API\Endpoints\v%d\NamespacedEndpointStub',
-        API::CURRENT_VERSION
-      )
-    );
+
+    $response = $this->api->getRequestData($data);
+    expect($response->status)->equals(APIResponse::STATUS_BAD_REQUEST);
   }
 
   function testItAcceptsAndProcessesAPIVersion() {
-    $namespace = "MailPoet\\Some\\Name\\Space\\Endpoints";
+    $namespace = array(
+      'name' => 'MailPoet\API\Endpoints',
+      'version' => 'v2'
+    );
     $this->api->addEndpointNamespace($namespace);
 
     $data = array(
       'endpoint' => 'namespaced_endpoint_stub',
-      'api_version' => 2,
+      'api_version' => 'v2',
       'method' => 'test'
     );
     $this->api->getRequestData($data);
@@ -102,12 +116,16 @@ class APITest extends MailPoetTest {
   }
 
   function testItCallsAddedEndpoints() {
-    $namespace = "MailPoet\\Some\\Name\\Space\\Endpoints";
+    $namespace = array(
+      'name' => 'MailPoet\API\Endpoints',
+      'version' => 'v1'
+    );
     $this->api->addEndpointNamespace($namespace);
 
     $data = array(
       'endpoint' => 'namespaced_endpoint_stub',
       'method' => 'test',
+      'api_version' => 'v1',
       'data' => array('test' => 'data')
     );
     $this->api->getRequestData($data);
@@ -117,18 +135,21 @@ class APITest extends MailPoetTest {
   }
 
   function testItCallsAddedEndpointsForSpecificAPIVersion() {
-    $namespace = "MailPoet\\Some\\Name\\Space\\Endpoints";
+    $namespace = array(
+      'name' => 'MailPoet\API\Endpoints',
+      'version' => 'v2'
+    );
     $this->api->addEndpointNamespace($namespace);
 
     $data = array(
       'endpoint' => 'namespaced_endpoint_stub',
-      'api_version' => 2,
+      'api_version' => 'v2',
       'method' => 'testVersion'
     );
     $this->api->getRequestData($data);
     $response = $this->api->processRoute();
 
-    expect($response->getData()['data'])->equals('version_test_succeeded');
+    expect($response->getData()['data'])->equals($data['api_version']);
   }
 
   function _after() {
diff --git a/tests/unit/API/APITestNamespacedEndpointStubV2.php b/tests/unit/API/APITestNamespacedEndpointStubV2.php
index 96824812ac..5e00ba8e75 100644
--- a/tests/unit/API/APITestNamespacedEndpointStubV2.php
+++ b/tests/unit/API/APITestNamespacedEndpointStubV2.php
@@ -13,6 +13,6 @@ class NamespacedEndpointStub extends APIEndpoint {
   );
 
   function testVersion() {
-    return $this->successResponse('version_test_succeeded');
+    return $this->successResponse('v2');
   }
 }
diff --git a/views/form/editor.html b/views/form/editor.html
index 970ca17ecb..60593ede98 100644
--- a/views/form/editor.html
+++ b/views/form/editor.html
@@ -275,6 +275,7 @@
         var template = Handlebars.compile($('#form_template_fields').html());
 
         return MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'customFields',
           action: 'getAll',
         }).done(function(response) {
@@ -403,6 +404,7 @@
           // save change if necessary
           if(new_value !== '' && current_value !== new_value) {
             MailPoet.Ajax.post({
+              api_version: window.mailpoet_api_version,
               endpoint: 'forms',
               action: 'save',
               data:  {
@@ -453,6 +455,7 @@
 
         function mailpoet_form_preview() {
           MailPoet.Ajax.post({
+            api_version: window.mailpoet_api_version,
             endpoint: 'forms',
             action: 'previewEditor',
             data: WysijaForm.save()
@@ -496,6 +499,7 @@
           } else {
             // save form
             MailPoet.Ajax.post({
+              api_version: window.mailpoet_api_version,
               endpoint: 'forms',
               action: 'saveEditor',
               data: form
@@ -550,6 +554,7 @@
         function mailpoet_form_export() {
           var template = Handlebars.compile($('#form_template_exports').html());
           MailPoet.Ajax.post({
+            api_version: window.mailpoet_api_version,
             endpoint: 'forms',
             action: 'exportsEditor',
             data: {
@@ -590,6 +595,7 @@
           var id = $(this).data('id');
 
           MailPoet.Ajax.post({
+            api_version: window.mailpoet_api_version,
             endpoint: 'customFields',
             action: 'get',
             data: {
@@ -621,6 +627,7 @@
             "<%= __('This field will be deleted for all your subscribers. Are you sure?') %>"
           )) {
             MailPoet.Ajax.post({
+              api_version: window.mailpoet_api_version,
               endpoint: 'customFields',
               action: 'delete',
               data: {
diff --git a/views/form/templates/settings/field_form.hbs b/views/form/templates/settings/field_form.hbs
index 90717a4729..61d6927d4e 100644
--- a/views/form/templates/settings/field_form.hbs
+++ b/views/form/templates/settings/field_form.hbs
@@ -74,6 +74,7 @@
 
         // save custom field
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'customFields',
           action: 'save',
           data: data
diff --git a/views/newsletter/editor.html b/views/newsletter/editor.html
index a982925cea..176d3f3a79 100644
--- a/views/newsletter/editor.html
+++ b/views/newsletter/editor.html
@@ -1201,6 +1201,7 @@
     };
 
     MailPoet.Ajax.post({
+      api_version: window.mailpoet_api_version,
       endpoint: 'newsletters',
       action: 'get',
       data: {
diff --git a/views/settings.html b/views/settings.html
index a3b4f9a93e..82f379e056 100644
--- a/views/settings.html
+++ b/views/settings.html
@@ -74,6 +74,7 @@
           MailPoet.Modal.loading(true);
 
           MailPoet.Ajax.post({
+            api_version: window.mailpoet_api_version,
             endpoint: 'settings',
             action: 'set',
             data: settings_data
diff --git a/views/settings/advanced.html b/views/settings/advanced.html
index 37c1896744..8f0efa6d6e 100644
--- a/views/settings/advanced.html
+++ b/views/settings/advanced.html
@@ -172,6 +172,7 @@
         )) {
           MailPoet.Modal.loading(true);
           MailPoet.Ajax.post({
+            'api_version': window.mailpoet_api_version,
             'endpoint': 'setup',
             'action': 'reset'
           }).always(function() {
diff --git a/views/settings/mta.html b/views/settings/mta.html
index f464c43649..1ed9a8e999 100644
--- a/views/settings/mta.html
+++ b/views/settings/mta.html
@@ -707,6 +707,7 @@
 
         MailPoet.Modal.loading(true);
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'mailer',
           action: 'send',
           data: {
@@ -751,6 +752,7 @@
 
         MailPoet.Modal.loading(true);
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: 'services',
           action: 'verifyMailPoetKey',
           data: {
diff --git a/views/welcome.html b/views/welcome.html
index d27f1a90db..1f0eba41ed 100644
--- a/views/welcome.html
+++ b/views/welcome.html
@@ -109,6 +109,7 @@ jQuery(function($) {
       $("#mailpoet_analytics_enabled").on("click", function() {
         var is_enabled = $(this).is(":checked") ? true : "";
         MailPoet.Ajax.post({
+          api_version: window.mailpoet_api_version,
           endpoint: "settings",
           action: "set",
           data: {