diff --git a/plugins/kolab_chat/config.inc.php.dist b/plugins/kolab_chat/config.inc.php.dist
index cc5fe534..d355d83b 100644
--- a/plugins/kolab_chat/config.inc.php.dist
+++ b/plugins/kolab_chat/config.inc.php.dist
@@ -8,6 +8,7 @@
     as in the Kolab server. Thanks to this we can auto-login users.
   1. It has to use the same domain, if it's using different we have to use a proxy:
     Following Apache config worked for me with kolab_chat_url=https://kolab.example.com/mattermost
+    Note: This should be simpler with Mattermost 5.1 (to be released soon).
 
     ProxyPreserveHost Off
     RewriteEngine On
@@ -31,3 +32,7 @@ $config['kolab_chat_url'] = 'https://mattermost.example.com';
 
 // Enables opening chat in a new window (or tab)
 $config['kolab_chat_extwin'] = false;
+
+// Default channel to select when opening the chat app.
+// Note: This has to be a channel ID.
+$config['kolab_chat_channel'] = null;
diff --git a/plugins/kolab_chat/drivers/mattermost.php b/plugins/kolab_chat/drivers/mattermost.php
index f9bb8b29..a5b572d8 100644
--- a/plugins/kolab_chat/drivers/mattermost.php
+++ b/plugins/kolab_chat/drivers/mattermost.php
@@ -55,9 +55,13 @@ class kolab_chat_mattermost
             $url .= '/api/v4/websocket';
         }
         else if ($this->rc->action == 'index' && $this->rc->task == 'kolab-chat') {
-            if ($channel = rcube_utils::get_input_value('_channel', rcube_utils::INPUT_GET)) {
-                $team    = rcube_utils::get_input_value('_team', rcube_utils::INPUT_GET);
+            $channel = rcube_utils::get_input_value('_channel', rcube_utils::INPUT_GET);
+            if (empty($channel)) {
+                $channel = $this->rc->config->get('kolab_chat_channel');
+            }
 
+            if ($channel) {
+                $team = rcube_utils::get_input_value('_team', rcube_utils::INPUT_GET);
                 if (empty($team) && ($_channel = $this->get_channel($channel, true))) {
                     $team = $_channel['team_name'];
                 }
@@ -101,6 +105,49 @@ class kolab_chat_mattermost
         exit;
     }
 
+    /**
+     * Handler for preferences_list hook.
+     * Adds options blocks into Chat settings sections in Preferences.
+     *
+     * @param array &$p Hook parameters
+     */
+    public function preferences_list(&$p)
+    {
+        $no_override = array_flip((array) $this->rc->config->get('dont_override'));
+
+        if (!isset($no_override['kolab_chat_channel'])) {
+            $field_id = 'rcmfd_kolab_chat_channel';
+            $select   = new html_select(array('name' => '_kolab_chat_channel', 'id' => $field_id));
+
+            $select->add('', '');
+            if ($channels = $this->get_channels_list()) {
+                foreach ($channels as $channel) {
+                    if ($channel['name'] && $channel['display_name']) {
+                        $select->add($channel['display_name'], $channel['id']);
+                    }
+                }
+            }
+
+            $p['blocks']['main']['options']['kolab_chat_channel'] = array(
+                'title'   => html::label($field_id, rcube::Q($this->plugin->gettext('defaultchannel'))),
+                'content' => $select->show($this->rc->config->get('kolab_chat_channel')),
+            );
+        }
+    }
+
+    /**
+     * Handler for preferences_save hook.
+     * Executed on Chat settings form submit.
+     *
+     * @param array &$p Hook parameters
+     */
+    public function preferences_save(&$p)
+    {
+        if (isset($_POST['_kolab_chat_channel'])) {
+            $p['prefs']['kolab_chat_channel'] = rcube_utils::get_input_value('_kolab_chat_channel', rcube_utils::INPUT_POST);
+        }
+    }
+
     /**
      * Returns the Mattermost session token
      * Note: This works only if the user/pass is the same in Kolab and Mattermost
@@ -122,7 +169,7 @@ class kolab_chat_mattermost
             }
 
             try {
-                $request = $this->get_api_request('GET', '/api/v4/users/me');
+                $request = $this->get_api_request('GET', '/users/me');
                 $request->setHeader('Authorization', "Bearer $token");
 
                 $response = $request->send();
@@ -140,7 +187,7 @@ class kolab_chat_mattermost
 
         // Request a new session token
         if (empty($token)) {
-            $request = $this->get_api_request('POST', '/api/v4/users/login');
+            $request = $this->get_api_request('POST', '/users/login');
             $request->setBody(json_encode(array(
                     'login_id' => $user,
                     'password' => $pass,
@@ -198,11 +245,11 @@ class kolab_chat_mattermost
      */
     protected function get_channel($channel_id, $extended = false)
     {
-        $channel = $this->api_get('/api/v4/channels/' . urlencode($channel_id));
+        $channel = $this->api_get('/channels/' . urlencode($channel_id));
 
         if ($extended && is_array($channel)) {
             if (!empty($channel['team_id'])) {
-                 $team = $this->api_get('/api/v4/teams/' . urlencode($channel['team_id']));
+                 $team = $this->api_get('/teams/' . urlencode($channel['team_id']));
             }
             else if ($teams = $this->get_user_teams()) {
                 // if there's no team assigned to the channel, we'll get the user's team
@@ -219,6 +266,27 @@ class kolab_chat_mattermost
         return $channel;
     }
 
+    /**
+     * Returns the Mattermost channels list for the user
+     *
+     * @return array Channels list (id, name, display_name, etc.)
+     */
+    protected function get_channels_list()
+    {
+        // FIXME: Should we list only public channels here?
+        // FIXME: SHould we check all user teams?
+
+        $user_id = $this->get_user_id();
+        $teams   = $this->get_user_teams();
+
+        if (!empty($teams) && ($team = $teams[0])) {
+            // FIXME: Why it does return channels not assigned to the specified team?
+            return $this->api_get(sprintf('/users/%s/teams/%s/channels', urlencode($user_id), urlencode($team['id'])));
+        }
+
+        return false;
+    }
+
     /**
      * Returns the Mattermost teams of the user
      *
@@ -226,7 +294,7 @@ class kolab_chat_mattermost
      */
     protected function get_user_teams()
     {
-        return $this->api_get('/api/v4/users/' . urlencode($this->get_user_id()) . '/teams');
+        return $this->api_get('/users/' . urlencode($this->get_user_id()) . '/teams');
     }
 
     /**
@@ -242,7 +310,7 @@ class kolab_chat_mattermost
 
         $config = array_merge($defaults, (array) $this->rc->config->get('kolab_chat_http_request'));
 
-        return libkolab::http_request($url . $path, $type, $config);
+        return libkolab::http_request($url . '/api/v4' . $path, $type, $config);
     }
 
     /**
diff --git a/plugins/kolab_chat/kolab_chat.php b/plugins/kolab_chat/kolab_chat.php
index 03f5e35d..830eaa4e 100644
--- a/plugins/kolab_chat/kolab_chat.php
+++ b/plugins/kolab_chat/kolab_chat.php
@@ -46,7 +46,6 @@ class kolab_chat extends rcube_plugin
             return;
         }
 
-        $this->add_texts('localization/');
         $this->load_config();
 
         $extwin = $this->rc->config->get('kolab_chat_extwin');
@@ -62,6 +61,8 @@ class kolab_chat extends rcube_plugin
         $class_name = "kolab_chat_$driver";
         $this->driver = new $class_name($this);
 
+        $this->add_texts('localization/');
+
         // Register UI end-points
         $this->register_task('kolab-chat');
         $this->register_action('index', array($this, 'ui'));
@@ -106,27 +107,25 @@ class kolab_chat extends rcube_plugin
      */
     function ui()
     {
-        if ($this->driver) {
-            $this->driver->ui();
+        $this->driver->ui();
 
-            $url = rcube::JQ($this->driver->url());
+        $url = rcube::JQ($this->driver->url());
 
-            if (!empty($_GET['redirect'])) {
-                echo '<!DOCTYPE html><html><head>'
-                    . '<meta http-equiv="refresh" content="0; url=' . $url . '">'
-                    . '</head><body></body></html>';
-                exit;
-            }
-            else {
-                $this->rc->output->add_script(
-                    "rcmail.addEventListener('init', function() {"
-                        . "rcmail.location_href('$url', rcmail.get_frame_window(rcmail.env.contentframe));"
-                    . "});",
-                    'foot'
-                );
+        if (!empty($_GET['redirect'])) {
+            echo '<!DOCTYPE html><html><head>'
+                . '<meta http-equiv="refresh" content="0; url=' . $url . '">'
+                . '</head><body></body></html>';
+            exit;
+        }
+        else {
+            $this->rc->output->add_script(
+                "rcmail.addEventListener('init', function() {"
+                    . "rcmail.location_href('$url', rcmail.get_frame_window(rcmail.env.contentframe));"
+                . "});",
+                'foot'
+            );
 
-                $this->rc->output->send('kolab_chat.chat');
-            }
+            $this->rc->output->send('kolab_chat.chat');
         }
     }
 
@@ -135,9 +134,7 @@ class kolab_chat extends rcube_plugin
      */
     function action()
     {
-        if ($this->driver) {
-            $this->driver->action();
-        }
+        $this->driver->action();
     }
 
     /**
@@ -193,6 +190,9 @@ class kolab_chat extends rcube_plugin
         if ($p['current']) {
             // update env flag in the parent window
             $this->rc->output->command('parent.set_env', array('kolab_chat_extwin' => (bool) $this->rc->config->get('kolab_chat_extwin')));
+
+            // Add driver-specific options
+            $this->driver->preferences_list($p);
         }
 
         return $p;
@@ -212,6 +212,9 @@ class kolab_chat extends rcube_plugin
             $p['prefs'] = array(
                 'kolab_chat_extwin' => isset($_POST['_kolab_chat_extwin']),
             );
+
+            // Add driver-specific options
+            $this->driver->preferences_save($p);
         }
 
         return $p;
diff --git a/plugins/kolab_chat/localization/en_US.inc b/plugins/kolab_chat/localization/en_US.inc
index 7447e557..7543f6e0 100644
--- a/plugins/kolab_chat/localization/en_US.inc
+++ b/plugins/kolab_chat/localization/en_US.inc
@@ -10,6 +10,7 @@
 
 $labels['chat'] = 'Chat';
 $labels['showinextwin'] = 'Open chat application in a new window';
+$labels['defaultchannel'] = 'Open chat application on channel';
 $labels['directmessage'] = 'A direct message from $u.';
 $labels['mentionmessage'] = 'You have been mentioned by $u in $c.';
 $labels['openchat'] = 'Open chat';