diff --git a/NEWS b/NEWS
index 1019b7973..ab1d00fca 100644
--- a/NEWS
+++ b/NEWS
@@ -9,6 +9,7 @@ Enhancements
  - [web] add Indonesian (id) translation
  - [web] updated Angular Material to version 1.1.19
  - [web] replaced bower packages by npm packages
+ - [web] restored mail threads (#3478, #4616, #4735)
 
 Bug fixes
  - [web] fixed wrong translation of custom calendar categories
diff --git a/UI/MailerUI/UIxMailActions.m b/UI/MailerUI/UIxMailActions.m
index 688b22073..2d5329677 100644
--- a/UI/MailerUI/UIxMailActions.m
+++ b/UI/MailerUI/UIxMailActions.m
@@ -18,6 +18,7 @@
  * Boston, MA 02111-1307, USA.
  */
 
+#import <Foundation/NSURL.h>
 #import <Foundation/NSValue.h>
 
 #import <NGObjWeb/WOContext+SoObjects.h>
@@ -221,16 +222,30 @@
 
 - (void) collapseAction: (BOOL) isCollapsing
 {
-  WORequest *request;
-  NSMutableDictionary *moduleSettings, *threadsCollapsed, *content;
+  NSArray *currentComponents;
   NSMutableArray *mailboxThreadsCollapsed;
-  NSString *msguid, *keyForMsgUIDs;
+  NSMutableDictionary *moduleSettings, *threadsCollapsed;
+  NSString *accountName, *msguid, *keyForMsgUIDs;
+  SOGoMailAccount *account;
+  SOGoMailFolder *mailbox;
+  SOGoMailObject *co;
   SOGoUserSettings *us;
+  int count;
 
-  request = [context request];
-  content = [[request contentAsString] objectFromJSONString];
-  keyForMsgUIDs = [content objectForKey:@"currentMailbox"];
-  msguid = [content objectForKey:@"msguid"];
+  co = [self clientObject];
+  account = [co mailAccountFolder];
+  accountName = [account nameInContainer];
+  mailbox = [co container];
+  msguid = [co nameInContainer];
+
+  // Build lookup key for current mailbox path
+  currentComponents = [[mailbox imap4URL] pathComponents];
+  count = [currentComponents count];
+  currentComponents = [[currentComponents subarrayWithRange: NSMakeRange(1,count-1)]
+                                resultsOfSelector: @selector (asCSSIdentifier)];
+  currentComponents = [currentComponents stringsWithFormat: @"folder%@"];
+  keyForMsgUIDs = [NSString stringWithFormat:@"/%@/%@", accountName,
+                            [currentComponents componentsJoinedByString: @"/"]];
 
   us = [[context activeUser] userSettings];
   if (!(moduleSettings = [us objectForKey: @"Mail"]))
diff --git a/UI/Templates/MailerUI/UIxMailFolderTemplate.wox b/UI/Templates/MailerUI/UIxMailFolderTemplate.wox
index 20e117604..e958d9ac9 100644
--- a/UI/Templates/MailerUI/UIxMailFolderTemplate.wox
+++ b/UI/Templates/MailerUI/UIxMailFolderTemplate.wox
@@ -300,7 +300,7 @@
           <md-list-item
               aria-label="{{currentMessage.subject}}"
               class="sg-message-list-item"
-              ng-class="{'md-default-theme md-accent md-bg md-hue-2': mailbox.selectedFolder.isSelectedMessage(currentMessage.uid, currentMessage.$mailbox.path)}"
+              ng-class="{'md-default-theme md-accent md-bg md-hue-2': mailbox.selectedFolder.isSelectedMessage(currentMessage.uid, currentMessage.$mailbox.path), 'sg-message-thread-first': currentMessage.first, 'sg-message-thread': currentMessage.threadMember}"
               md-virtual-repeat="currentMessage in mailbox.service.selectedFolder"
               md-on-demand="md-on-demand" md-item-size="56"
               ng-click="mailbox.selectMessage(currentMessage)"
diff --git a/UI/Templates/PreferencesUI/UIxPreferences.wox b/UI/Templates/PreferencesUI/UIxPreferences.wox
index b202e42f0..ea4f0431c 100644
--- a/UI/Templates/PreferencesUI/UIxPreferences.wox
+++ b/UI/Templates/PreferencesUI/UIxPreferences.wox
@@ -578,7 +578,7 @@
               </md-checkbox>
             </div>
 
-            <div class="ng-hide">
+            <div>
               <md-checkbox
                   ng-model="app.preferences.defaults.SOGoMailSortByThreads"
                   ng-true-value="1"
diff --git a/UI/WebServerResources/js/Common/Common.app.js b/UI/WebServerResources/js/Common/Common.app.js
index 1df505957..eda8f0f24 100644
--- a/UI/WebServerResources/js/Common/Common.app.js
+++ b/UI/WebServerResources/js/Common/Common.app.js
@@ -266,6 +266,16 @@
       '}'
     ].join(''));
 
+    // Register custom stylesheet for Mail module
+    $mdThemingProvider.registerStyles([
+      '.sg-message-thread {',
+      '  background-color: \'{{primary-100}}\';',
+      '}',
+      '.sg-message-thread-first {',
+      '  background-color: \'{{primary-200}}\';',
+      '}',
+    ].join(''));
+
     if (!window.DebugEnabled) {
       // Disable debugging information
       $logProvider.debugEnabled(false);
diff --git a/UI/WebServerResources/js/Mailer/Mailbox.service.js b/UI/WebServerResources/js/Mailer/Mailbox.service.js
index 1fd90b667..d3f837b07 100644
--- a/UI/WebServerResources/js/Mailer/Mailbox.service.js
+++ b/UI/WebServerResources/js/Mailer/Mailbox.service.js
@@ -165,6 +165,9 @@
     if (this.path) {
       this.id = this.$id();
       this.$acl = new Mailbox.$$Acl('Mail/' + this.id);
+      if (this.threaded) {
+        this.$collapsedThreads = Mailbox.$Preferences.settings.Mail.threadsCollapsed['/' + this.id] || [];
+      }
     }
     this.$displayName = this.name;
     if (this.type) {
@@ -222,7 +225,16 @@
    * @returns the number of messages in the mailbox
    */
   Mailbox.prototype.getLength = function() {
-    return this.$messages.length;
+    var _this = this, collapsedThread = false;
+    var visibleMessages = _.filter(this.$messages, function(msg, i) {
+      if (msg.first) {
+        collapsedThread = msg.collapsed;
+      } else if (msg.level < 0) {
+        collapsedThread = false;
+      }
+      return msg.first || collapsedThread === false;
+    });
+    return visibleMessages.length;
   };
 
   /**
@@ -232,10 +244,18 @@
    * @returns the message at the specified index
    */
   Mailbox.prototype.getItemAtIndex = function(index) {
-    var message;
+    var _this = this, collapsedThread = false, message;
+    var visibleMessages = _.filter(this.$messages, function(msg, i) {
+      if (msg.first) {
+        collapsedThread = msg.collapsed;
+      } else if (msg.level < 0) {
+        collapsedThread = false; // leaving the thread
+      }
+      return msg.first || collapsedThread === false;
+    });
 
-    if (index >= 0 && index < this.$messages.length) {
-      message = this.$messages[index];
+    if (index >= 0 && index < visibleMessages.length) {
+      message = visibleMessages[index];
       this.$lastVisibleIndex = Math.max(0, index - 3); // Magic number is NUM_EXTRA from virtual-repeater.js
 
       if (this.$loadMessage(message.uid))
@@ -928,7 +948,7 @@
 
           // First entry of 'uids' are keys when threaded view is enabled
           if (_this.threaded) {
-            uids = _this.uids[0];
+            uids = _this.uids[0]; // uid, level, first
             _this.uids.splice(0, 1);
           }
 
@@ -937,6 +957,19 @@
             var data, msgObject;
             if (_this.threaded) {
               data = _.zipObject(uids, msg);
+              if (data.first === 1) {
+                var count = 1;
+                while (_this.uids[i + count] &&
+                       _this.uids[i + count][1] >= 0 &&
+                       _this.uids[i + count][2] !== 1) {
+                  count++;
+                }
+                data.count = count;
+                data.collapsed = false;
+                if (_this.$collapsedThreads.indexOf(data.uid.toString()) >= 0) {
+                  data.collapsed = true;
+                }
+              }
             } else {
               data = {uid: msg.toString()};
             }
diff --git a/UI/WebServerResources/js/Mailer/Message.service.js b/UI/WebServerResources/js/Mailer/Message.service.js
index 94d6bfe90..880e14d6d 100644
--- a/UI/WebServerResources/js/Mailer/Message.service.js
+++ b/UI/WebServerResources/js/Mailer/Message.service.js
@@ -26,6 +26,15 @@
         this.init(futureMessageData);
       }
       this.uid = parseInt(futureMessageData.uid);
+      this.level = parseInt(futureMessageData.level);
+      this.first = parseInt(futureMessageData.first) === 1;
+      if (this.first) {
+        this.threadCount = parseInt(futureMessageData.count);
+        this.collapsed = (futureMessageData.collapsed === true);
+      }
+      else if (!isNaN(this.level) && this.level >= 0) {
+        this.threadMember = true;
+      }
     }
     else {
       // The promise will be unwrapped first
@@ -539,7 +548,7 @@
   };
 
   /**
-   * @function $markAsFlaggedOrUnflagged
+   * @function toggleFlag
    * @memberof Message.prototype
    * @desc Add or remove a the \\Flagged flag on the current message.
    * @returns a promise of the HTTP operation
@@ -558,6 +567,24 @@
     });
   };
 
+  /**
+   * @function toggleThread
+   * @memberof Message.prototype
+   * @desc Collapse or expand mail thread
+   * @returns a promise of the HTTP operation
+   */
+  Message.prototype.toggleThread = function() {
+    var _this = this,
+        action = 'markMessageCollapse';
+
+    if (this.collapsed)
+      action = 'markMessageUncollapse';
+
+    this.collapsed = !this.collapsed;
+
+    return Message.$$resource.post(this.$absolutePath(), action);
+  };
+
   /**
    * @function $isLoading
    * @memberof Message.prototype
diff --git a/UI/WebServerResources/js/Mailer/sgMessageListItemMain.directive.js b/UI/WebServerResources/js/Mailer/sgMessageListItemMain.directive.js
index 0c1d8aebb..1d365afd2 100644
--- a/UI/WebServerResources/js/Mailer/sgMessageListItemMain.directive.js
+++ b/UI/WebServerResources/js/Mailer/sgMessageListItemMain.directive.js
@@ -26,6 +26,9 @@
         '  <div class="sg-md-body">',
         '    <div class="sg-tile-subject"><!-- subject --></div>',
         '    <div class="sg-tile-size"><!-- size --></div>',
+        '    <md-button class="sg-tile-thread md-secondary ng-hide" md-colors="::{ color: \'accent-600\'}" ng-click="$ctrl.toggleThread()">',
+        '      <md-icon class="md-rotate-180-ccw" md-colors="::{ color: \'accent-600\'}">expand_more</md-icon><span><span>', // expanded by default (icon is rotated)
+        '    </md-button>',
         '  </div>',
         '</div>',
         '<div class="sg-tile-icons">',
@@ -59,7 +62,7 @@
     var $ctrl = this;
 
     this.$postLink = function () {
-      var contentDivElement, iconsDivElement;
+      var contentDivElement, threadButton, iconsDivElement;
       var parentControllerOnUpdate, setVisibility;
 
       this.parentController = $scope.parentController;
@@ -74,6 +77,12 @@
           iconsDivElement = angular.element(div);
       });
 
+      threadButton = contentDivElement.find('button')[0];
+      this.threadButton = threadButton;
+      threadButton = angular.element(threadButton);
+      this.threadIconElement = threadButton.find('md-icon')[0];
+      this.threadCountElement = threadButton.find('span')[0];
+
       this.priorityIconElement = contentDivElement.find('md-icon')[0];
 
       if (Mailbox.$virtualMode) {
@@ -147,9 +156,19 @@
         else
           $ctrl.priorityIconElement.classList.add('ng-hide');
 
+        // Mail thread
+        if ($ctrl.message.first) {
+          $ctrl.threadButton.classList.remove('ng-hide');
+          $ctrl.threadCountElement.innerHTML = $ctrl.message.threadCount;
+          if ($ctrl.message.collapsed)
+            $ctrl.threadIconElement.classList.remove('md-rotate-180-ccw');
+        }
+        else {
+          $ctrl.threadButton.classList.add('ng-hide');
+        }
+
         // Subject
         $ctrl.subjectElement.innerHTML = $ctrl.message.subject.encodeEntities();
-
         // Message size
         $ctrl.sizeElement.innerHTML = $ctrl.message.size;
 
@@ -173,6 +192,14 @@
       this.MailboxService = Mailbox;
     };
 
+    this.toggleThread = function() {
+      if (this.message.collapsed)
+        this.threadIconElement.classList.add('md-rotate-180-ccw');
+      else
+        this.threadIconElement.classList.remove('md-rotate-180-ccw');
+      this.message.toggleThread();
+    };
+
   }
 
 
diff --git a/UI/WebServerResources/js/Preferences/Preferences.service.js b/UI/WebServerResources/js/Preferences/Preferences.service.js
index 2f5fd9158..d174f4db7 100644
--- a/UI/WebServerResources/js/Preferences/Preferences.service.js
+++ b/UI/WebServerResources/js/Preferences/Preferences.service.js
@@ -11,7 +11,7 @@
     var _this = this, defaultsElement, settingsElement, data;
 
     this.defaults = {};
-    this.settings = {};
+    this.settings = {Mail: {}};
 
     defaultsElement = Preferences.$document[0].getElementById('UserDefaults');
     if (defaultsElement) {
diff --git a/UI/WebServerResources/scss/components/list/list.scss b/UI/WebServerResources/scss/components/list/list.scss
index b64ab2d49..438a1611b 100644
--- a/UI/WebServerResources/scss/components/list/list.scss
+++ b/UI/WebServerResources/scss/components/list/list.scss
@@ -172,16 +172,25 @@ div.md-tile-left {
   &-tile-content {
     flex: 1;
     overflow: hidden; // required in Firefox
-    .sg-tile-date, .sg-tile-size {
+    .sg-tile-date, .sg-tile-size, .sg-tile-thread {
       flex-shrink: 0;
       font-size: sg-size(body);
       font-weight: $sg-font-light;
       line-height: $sg-line-height-2;
-      margin-left: 3px;
+      margin-left: 3px !important;
     }
     .sg-tile-size {
       font-size: sg-size(caption);
     }
+    .sg-tile-thread {
+      min-height: auto;
+      min-width: auto;
+      padding: 0 3px !important;
+      font-weight: $sg-font-medium;
+      md-icon {
+        font-size: sg-size(body);
+      }
+    }
     .#{$md}-subhead {
       @extend .#{$md}-body-1;
       white-space: pre;