diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -438,6 +438,11 @@
                 break;
 
             case 'free_busy':
+                // Outlook sets the busy state to the attendance state, and we don't want to change the event state based on that.
+                // Outlook doesn't have the concept of an event state, so we just ignore this.
+                if ($is_outlook) {
+                    continue 2;
+                }
                 $map   = array_flip($this->busyStatusMap);
                 $value = isset($map[$value]) ? $map[$value] : null;
                 break;
@@ -545,6 +550,22 @@
             }
         }
 
+        // Outlook does not send the correct attendee status when changing between accepted and tentative, but it toggles the busyStatus.
+        if ($is_outlook) {
+            $status = null;
+            if ($data->busyStatus == self::BUSY_STATUS_BUSY) {
+                $status = "ACCEPTED";
+            } else if ($data->busyStatus == self::BUSY_STATUS_TENTATIVE) {
+                $status = "TENTATIVE";
+            }
+
+            if ($status) {
+                $this->logger->debug("Updating our attendee status based on the busy status to {$status}.");
+                $emails = $this->user_emails();
+                $this->find_and_update_attendee_status($attendees, $status, $emails);
+            }
+        }
+
         if (!$is_exception) {
             // Make sure the event has the organizer set
             if (!$organizer_email && ($identity = kolab_sync::get_instance()->user->get_identity())) {
@@ -560,6 +581,7 @@
         }
 
         $event['attendees']  = $attendees;
+
         $event['categories'] = $categories;
         $event['exceptions'] = isset($event['recurrence']['EXCEPTIONS']) ? $event['recurrence']['EXCEPTIONS'] : array();
 
@@ -809,25 +831,33 @@
     }
 
     /**
-     * Update the attendee status of the user
+     * Update the attendee status of the user matching $emails
      */
-    protected function update_attendee_status(&$event, $status)
+    protected function find_and_update_attendee_status(&$attendees, $status, $emails)
     {
-        $emails = $this->user_emails();
-
-        foreach ((array) $event['attendees'] as $i => $attendee) {
+        $found = false;
+        foreach ((array) $attendees as $i => $attendee) {
             if (!empty($attendee['email'])
                 && (empty($attendee['role']) || $attendee['role'] != 'ORGANIZER')
                 && in_array_nocase($attendee['email'], $emails)
             ) {
-                $event['attendees'][$i]['status'] = $status;
-                $event['attendees'][$i]['rsvp']   = false;
-                $event_attendee = $attendee;
+                $attendees[$i]['status'] = $status;
+                $attendees[$i]['rsvp']   = false;
                 $this->logger->debug('Updating existing attendee: ' . $attendee['email'] . ' status: ' . $status);
+                $found = true;
             }
         }
+        return $found;
+    }
+
+    /**
+     * Update the attendee status of the user
+     */
+    protected function update_attendee_status(&$event, $status)
+    {
+        $emails = $this->user_emails();
 
-        if (empty($event_attendee)) {
+        if (!$this->find_and_update_attendee_status($event['attendees'], $status, $emails)) {
             $this->logger->debug('Adding new attendee ' . $emails[0] . ' status: ' . $status);
             // Add the user to the attendees list
             $event['attendees'][] = array(