diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -3437,39 +3437,38 @@
 
                     // only update attendee status
                     if ($event['_method'] == 'REPLY') {
-                        // try to identify the attendee using the email sender address
-                        $existing_attendee        = -1;
-                        $existing_attendee_emails = [];
-
-                        foreach ($existing['attendees'] as $i => $attendee) {
-                            $existing_attendee_emails[] = $attendee['email'];
-                            if ($this->itip->compare_email($attendee['email'], $event['_sender'], $event['_sender_utf'])) {
-                                $existing_attendee = $i;
-                            }
-                        }
+                        $existing_attendee_index  = -1;
 
                         $event_attendee   = null;
                         $update_attendees = [];
 
-                        foreach ($event['attendees'] as $attendee) {
-                            if ($this->itip->compare_email($attendee['email'], $event['_sender'], $event['_sender_utf'])) {
-                                $event_attendee       = $attendee;
-                                $update_attendees[]   = $attendee;
-                                $metadata['fallback'] = $attendee['status'];
-                                $metadata['attendee'] = $attendee['email'];
-                                $metadata['rsvp']     = !empty($attendee['rsvp']) || $attendee['role'] != 'NON-PARTICIPANT';
+                        if ($attendee = $this->itip->find_reply_attendee($event)) {
+                            $event_attendee       = $attendee;
+                            $update_attendees[]   = $attendee;
+                            $metadata['fallback'] = $attendee['status'];
+                            $metadata['attendee'] = $attendee['email'];
+                            $metadata['rsvp']     = !empty($attendee['rsvp']) || $attendee['role'] != 'NON-PARTICIPANT';
 
-                                if ($attendee['status'] != 'DELEGATED') {
-                                    break;
+                            $existing_attendee_emails = [];
+
+                            // Find the attendee to update
+                            foreach ($existing['attendees'] as $i => $existing_attendee) {
+                                $existing_attendee_emails[] = $existing_attendee['email'];
+                                if ($this->itip->compare_email($existing_attendee['email'], $attendee['email'])) {
+                                    $existing_attendee_index = $i;
                                 }
                             }
-                            // also copy delegate attendee
-                            else if (!empty($attendee['delegated-from'])
-                                && $this->itip->compare_email($attendee['delegated-from'], $event['_sender'], $event['_sender_utf'])
-                            ) {
-                                $update_attendees[] = $attendee;
-                                if (!in_array_nocase($attendee['email'], $existing_attendee_emails)) {
-                                    $existing['attendees'][] = $attendee;
+
+                            if ($attendee['status'] == 'DELEGATED') {
+                                //Also find and copy the delegatee
+                                $delegatee_email = $attendee['email'];
+                                $delegatees = array_filter($event['attendees'], function($attendee) use ($delegatee_email){ return $attendee['role'] != 'ORGANIZER' && $this->itip->compare_email($attendee['delegated-from'], $delegatee_email); });
+
+                                if ($delegatee = $this->itip->find_attendee_by_email($event['attendees'], 'delegated-from', $attendee['email'])) {
+                                    $update_attendees[] = $delegatee;
+                                    if (!in_array_nocase($delegatee['email'], $existing_attendee_emails)) {
+                                        $existing['attendees'][] = $delegated_attendee;
+                                    }
                                 }
                             }
                         }
@@ -3487,29 +3486,9 @@
                             }
                         }
 
-                        // Accept sender as a new participant (different email in From: and the iTip)
-                        // Use ATTENDEE entry from the iTip with replaced email address
-                        if (!$event_attendee) {
-                            // remove the organizer
-                            $itip_attendees = array_filter(
-                                $event['attendees'],
-                                function($item) { return $item['role'] != 'ORGANIZER'; }
-                            );
-
-                            // there must be only one attendee
-                            if (is_array($itip_attendees) && count($itip_attendees) == 1) {
-                                $event_attendee          = $itip_attendees[key($itip_attendees)];
-                                $event_attendee['email'] = $event['_sender'];
-                                $update_attendees[]      = $event_attendee;
-                                $metadata['fallback']    = $event_attendee['status'];
-                                $metadata['attendee']    = $event_attendee['email'];
-                                $metadata['rsvp']        = !empty($event_attendee['rsvp']) || $event_attendee['role'] != 'NON-PARTICIPANT';
-                            }
-                        }
-
                         // found matching attendee entry in both existing and new events
-                        if ($existing_attendee >= 0 && $event_attendee) {
-                            $existing['attendees'][$existing_attendee] = $event_attendee;
+                        if ($existing_attendee_index >= 0 && $event_attendee) {
+                            $existing['attendees'][$existing_attendee_index] = $event_attendee;
                             $success = $this->driver->update_attendees($existing, $update_attendees);
                         }
                         // update the entire attendees block
diff --git a/plugins/libcalendaring/lib/libcalendaring_itip.php b/plugins/libcalendaring/lib/libcalendaring_itip.php
--- a/plugins/libcalendaring/lib/libcalendaring_itip.php
+++ b/plugins/libcalendaring/lib/libcalendaring_itip.php
@@ -633,30 +633,13 @@
         if ($method == 'REPLY') {
             $title = $this->gettext('itipreply');
 
-            foreach ($event['attendees'] as $attendee) {
-                if (!empty($attendee['email']) && $attendee['role'] != 'ORGANIZER') {
-                    if (empty($event['_sender']) || self::compare_email($attendee['email'], $event['_sender'], $event['_sender_utf'])) {
-                        $metadata['attendee'] = $attendee['email'];
-                        $rsvp_status = strtoupper($attendee['status']);
-                        if ($attendee['delegated-to']) {
-                            $metadata['delegated-to'] = $attendee['delegated-to'];
-                        }
-                        break;
-                    }
-                }
-            }
-
-            // It may happen that sender's address is different in From: and the attached iTip
-            // In such case use the ATTENDEE entry with the address from From: header
-            if (empty($metadata['attendee']) && !empty($event['_sender'])) {
-                // remove the organizer
-                $itip_attendees = array_filter($event['attendees'], function($item) { return $item['role'] != 'ORGANIZER'; });
+            $attendee = self::find_reply_attendee($event);
 
-                // there must be only one attendee
-                if (is_array($itip_attendees) && count($itip_attendees) == 1) {
-                    $event_attendee       = $itip_attendees[key($itip_attendees)];
-                    $metadata['attendee'] = $event['_sender'];
-                    $rsvp_status          = strtoupper($event_attendee['status']);
+            if ($attendee) {
+                $metadata['attendee'] = $attendee['email'];
+                $rsvp_status = strtoupper($attendee['status']);
+                if ($attendee['delegated-to']) {
+                    $metadata['delegated-to'] = $attendee['delegated-to'];
                 }
             }
 
@@ -1013,4 +996,50 @@
 
         return $v1 || $v2;
     }
+
+    /**
+     * Find an attendee that is not the organizer and has an email matching $email_field
+     */
+    public function find_attendee_by_email($attendees, $email_field, $email, $email_utf = null) {
+        foreach ($attendees as $_attendee) {
+            if ($attendee['role'] == 'ORGANIZER') {
+                continue;
+            }
+            if (!empty($attendee[$email_field]) && self::compare_email($attendee[$email_field], $email, $email_utf)) {
+                return $attendee;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Find the replying attendee in a REPLY
+     */
+    public static function find_reply_attendee($event) {
+        // remove the organizer
+        $itip_attendees = array_filter($event['attendees'], function($item) { return $item['role'] != 'ORGANIZER' && !empty($item['email']); });
+        $attendee = null;
+
+        // According to rfc there should only be one attendee for a REPLY
+        if (count($itip_attendees) == 1) {
+            return array_pop($itip_attendees);
+        }
+
+        // If we don't have anything to match by, pick the first and hope for the best.
+        if (empty($event['_sender'])) {
+            return array_shift($itip_attendees);
+        }
+
+        // try to match by sent-by
+        if ($attendee = self::find_attendee_by_email($itip_attendees, 'sent-by', $event['_sender'], $event['_sender_utf'])) {
+            return $attendee;
+        }
+
+        // try to match by email
+        if ($attendee = self::find_attendee_by_email($itip_attendees, 'email', $event['_sender'], $event['_sender_utf'])) {
+            return $attendee;
+        }
+
+        return null;
+    }
 }