diff --git a/NEWS b/NEWS
index 52680f470..588ed76f9 100644
--- a/NEWS
+++ b/NEWS
@@ -3,7 +3,8 @@
 
 Enhancements
  - [web] create card from sender or recipient address (#3002, #4610)
- - [core] baseDN now accept dynamic domain values (#3685)
+ - [core] baseDN now accept dynamic domain values (#3685 - sponsored by iRedMail)
+ - [core] we now handle optional and non-required attendee states
 
 Bug fixes
  - [web] fixed all-day event dates with different timezone
diff --git a/SoObjects/Appointments/English.lproj/Localizable.strings b/SoObjects/Appointments/English.lproj/Localizable.strings
index da4f31192..03707ef52 100644
--- a/SoObjects/Appointments/English.lproj/Localizable.strings
+++ b/SoObjects/Appointments/English.lproj/Localizable.strings
@@ -62,3 +62,8 @@ vtodo_class2 = "(Confidential task)";
 /* Resources */
 "Cannot access resource: \"%{Cn} %{SystemEmail}\"" = "Cannot access resource: \"%{Cn} %{SystemEmail}\"";
 "Maximum number of simultaneous bookings (%{NumberOfSimultaneousBookings}) reached for resource \"%{Cn} %{SystemEmail}\". The conflicting event is \"%{EventTitle}\", and starts on %{StartDate}." = "Maximum number of simultaneous bookings (%{NumberOfSimultaneousBookings}) reached for resource \"%{Cn} %{SystemEmail}\". The conflicting event is \"%{EventTitle}\", and starts on %{StartDate}.";
+
+/* Participation role */
+"Your participation is required to this event" = "Your participation is required to this event";
+"Your participation is optional to this event" = "Your participation is optional to this event";
+"Your participation is not required to this event" = "Your participation is not required to this event";
\ No newline at end of file
diff --git a/SoObjects/Appointments/SOGoAppointmentObject.m b/SoObjects/Appointments/SOGoAppointmentObject.m
index a38789820..c02556d24 100644
--- a/SoObjects/Appointments/SOGoAppointmentObject.m
+++ b/SoObjects/Appointments/SOGoAppointmentObject.m
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2007-2016 Inverse inc.
+  Copyright (C) 2007-2019 Inverse inc.
 
   This file is part of SOGo
 
@@ -187,9 +187,17 @@
     {
       SOGoAppointmentObject *attendeeObject;
       iCalCalendar *iCalendarToSave;
-      
+      iCalPerson *attendee;
+      SOGoUser *user;
+
       iCalendarToSave = nil;
+      user = [SOGoUser userWithLogin: theUID];
       attendeeObject = [self _lookupEvent: [newEvent uid] forUID: theUID];
+      attendee = [newEvent userAsAttendee: user];
+
+      // If the atttende's role is NON-PARTICIPANT, we write nothing to its calendar
+      if ([[attendee role] caseInsensitiveCompare: @"NON-PARTICIPANT"] == NSOrderedSame)
+        return;
 
       if ([newEvent recurrenceId])
         {
@@ -197,12 +205,11 @@
           if ([attendeeObject isNew])
             {
               iCalEvent *ownerEvent;
-              SOGoUser *user;
+
               // We check if the attendee that was added to a single occurence is
               // present in the master component. If not, we create a calendar with
               // a single event for the occurence.
               ownerEvent = [[[newEvent parent] events] objectAtIndex: 0];
-              user = [SOGoUser userWithLogin: theUID];
 
               if (![ownerEvent userAsAttendee: user])
                 {
@@ -339,7 +346,7 @@
             }
         }
       else
-        [self errorWithFormat: @"Unable to find event with UID %@ in %@'s calendar - skipping delete operation", nameInContainer, theUID];
+        [self errorWithFormat: @"Unable to find event with UID %@ in %@'s calendar - skipping delete operation. This can be normal for NON-PARTICIPANT attendees.", nameInContainer, theUID];
     }
 }
 
@@ -1171,6 +1178,10 @@ inRecurrenceExceptionsForEvent: (iCalEvent *) theEvent
   NSException *error;
   BOOL addDelegate, removeDelegate;
 
+  // If the atttende's role is NON-PARTICIPANT, we write nothing to its calendar
+  if ([[attendee role] caseInsensitiveCompare: @"NON-PARTICIPANT"] == NSOrderedSame)
+    return;
+
   error = nil;
 
   eventObject = [self _lookupEvent: eventUID forUID: uid];
diff --git a/SoObjects/Appointments/SOGoAptMailInvitation.m b/SoObjects/Appointments/SOGoAptMailInvitation.m
index f3a449b1c..1343826ce 100644
--- a/SoObjects/Appointments/SOGoAptMailInvitation.m
+++ b/SoObjects/Appointments/SOGoAptMailInvitation.m
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2010-2012 Inverse
+  Copyright (C) 2010-2019 Inverse
 
   This file is part of SOGo
 
diff --git a/SoObjects/Appointments/SOGoAptMailNotification.h b/SoObjects/Appointments/SOGoAptMailNotification.h
index b650ee9e9..0b6add4d0 100644
--- a/SoObjects/Appointments/SOGoAptMailNotification.h
+++ b/SoObjects/Appointments/SOGoAptMailNotification.h
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2006-2018 Inverse inc.
+  Copyright (C) 2006-2019 Inverse inc.
 
   This file is part of SOGo.
 
@@ -28,6 +28,7 @@
 @class NSMutableDictionary;
 @class NSString;
 @class NSTimeZone;
+@class iCalPerson;
 @class iCalRepeatableEntityObject;
 @class SOGoDateFormatter;
 
@@ -46,6 +47,7 @@
   NSCalendarDate *oldEndDate;
   NSCalendarDate *newEndDate;
   NSString *organizerName;
+  iCalPerson *currentAttendee;
   NSMutableDictionary *values;
   SOGoDateFormatter *dateFormatter;
 }
@@ -61,6 +63,9 @@
 - (void) setOrganizerName: (NSString *) theString;
 - (NSString *) organizerName;
 
+- (void) setCurrentAttendee: (iCalPerson *) theAttendee;
+- (iCalPerson *) currentAttendee;
+
 - (NSCalendarDate *) oldStartDate;
 - (NSCalendarDate *) newStartDate;
 
diff --git a/SoObjects/Appointments/SOGoAptMailNotification.m b/SoObjects/Appointments/SOGoAptMailNotification.m
index a967f61ab..90a171040 100644
--- a/SoObjects/Appointments/SOGoAptMailNotification.m
+++ b/SoObjects/Appointments/SOGoAptMailNotification.m
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2006-2013 Inverse inc.
+  Copyright (C) 2006-2019 Inverse inc.
   Copyright (C) 2000-2005 SKYRIX Software AG
 
   This file is part of SOGo.
@@ -48,6 +48,8 @@
       apt = nil;
       values = nil;
       dateFormatter = RETAIN([[context activeUser] dateFormatterInContext: context]);
+      organizerName = nil;
+      currentAttendee = nil;
     }
 
   return self;
@@ -59,6 +61,7 @@
   [apt release];
   [previousApt release];
   [organizerName release];
+  [currentAttendee release];
   [viewTZ release];
   [oldStartDate release];
   [newStartDate release];
@@ -164,6 +167,16 @@
   return organizerName;
 }
 
+- (void) setCurrentAttendee: (iCalPerson *) theAttendee
+{
+  ASSIGN(currentAttendee, theAttendee);
+}
+
+- (iCalPerson *) currentAttendee
+{
+  return currentAttendee;
+}
+
 - (NSString *) sentByText
 {
   NSDictionary *sentByValues;
@@ -293,4 +306,21 @@
   return nil;
 }
 
+- (NSString *) getParticipationRole
+{
+  if ([[currentAttendee role] caseInsensitiveCompare: @"OPT-PARTICIPANT"] == NSOrderedSame)
+    {
+      return [self labelForKey: @"Your participation is optional to this event"
+                     inContext: context];
+    }
+  else if ([[currentAttendee role] caseInsensitiveCompare: @"NON-PARTICIPANT"] == NSOrderedSame)
+    {
+      return [self labelForKey: @"Your participation is not required to this event"
+                 inContext: context];
+    }
+
+  return [self labelForKey: @"Your participation is required to this event"
+                 inContext: context];
+}
+
 @end
diff --git a/SoObjects/Appointments/SOGoCalendarComponent.m b/SoObjects/Appointments/SOGoCalendarComponent.m
index 7650a4b00..d4e259943 100644
--- a/SoObjects/Appointments/SOGoCalendarComponent.m
+++ b/SoObjects/Appointments/SOGoCalendarComponent.m
@@ -1,6 +1,6 @@
 /* SOGoCalendarComponent.m - this file is part of SOGo
  *
- * Copyright (C) 2006-2016 Inverse inc.
+ * Copyright (C) 2006-2019 Inverse inc.
  *
  * This file is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -846,6 +846,7 @@
 		  p = [app pageWithName: pageName inContext: context];
 		  [p setApt: (iCalEvent *) object];
 		  [p setPreviousApt: (iCalEvent *) previousObject];
+                  [p setCurrentAttendee: attendee];
 		  
 		  if ([[object organizer] cn] && [[[object organizer] cn] length])
 		    {
@@ -893,9 +894,10 @@
 		  /* attach text part to multipart body */
 		  [body addBodyPart: bodyPart];
     
-		  /* attach calendar part to multipart body */
-		  [body addBodyPart: eventBodyPart];
-    
+		  /* attach calendar part to multipart body only if the participation role is not NON-PARTICIPANT */
+                  if ([[attendee role] caseInsensitiveCompare: @"NON-PARTICIPANT"] != NSOrderedSame)
+                    [body addBodyPart: eventBodyPart];
+
 		  /* attach multipart body to message */
 		  [msg setBody: body];
 		  [body release];
diff --git a/UI/Templates/Appointments/SOGoAptMailInvitation.wox b/UI/Templates/Appointments/SOGoAptMailInvitation.wox
index a95b52194..4664a86fa 100644
--- a/UI/Templates/Appointments/SOGoAptMailInvitation.wox
+++ b/UI/Templates/Appointments/SOGoAptMailInvitation.wox
@@ -20,7 +20,12 @@ th, td { font-family: Lucida Grande, Bitstream VeraSans, Tahoma, sans-serif; fon
           value="getSubject" const:escapeHTML="NO"/> <small style="font-size: 12px; color: #999;"><var:string
           value="organizerName" const:escapeHTML="NO"/><var:string value="sentByText" const:escapeHTML="NO"/></small></h1></td>
       </tr>
-      <var:if condition="location.length"
+      <tr>
+        <th></th>
+        <td><h1 style="font-size: 14px; font-weight: normal;
+          padding-bottom: 9px; border-bottom: 1px solid #ccc;"><var:string
+          value="getParticipationRole" const:escapeHTML="NO"/></h1></td>
+      </tr>  <var:if condition="location.length"
       ><tr>
         <th align="right" style="font-weight: bold;"><var:string label:value="location_label" const:escapeHTML="NO"/></th>
         <td><var:string value="location" const:escapeHTML="NO"/></td>
diff --git a/UI/Templates/Appointments/SOGoAptMailUpdate.wox b/UI/Templates/Appointments/SOGoAptMailUpdate.wox
index bce368fb7..99fac806b 100644
--- a/UI/Templates/Appointments/SOGoAptMailUpdate.wox
+++ b/UI/Templates/Appointments/SOGoAptMailUpdate.wox
@@ -20,6 +20,12 @@ th, td { font-family: Lucida Grande, Bitstream VeraSans, Tahoma, sans-serif; fon
         value="getSubject" const:escapeHTML="NO"/> <small style="font-size: 12px; color: #999;"><var:string
         value="organizerName" const:escapeHTML="NO"/><var:string value="sentByText" const:escapeHTML="NO"/></small></h1></td>
       </tr>
+      <tr>
+        <th></th>
+        <td><h1 style="font-size: 14px; font-weight: normal;
+          padding-bottom: 9px; border-bottom: 1px solid #ccc;"><var:string
+          value="getParticipationRole" const:escapeHTML="NO"/></h1></td>
+      </tr>
       <tr>
         <th></th>
         <td><var:string value="bodyStartText" const:escapeHTML="NO"/></td>