diff --git a/NEWS b/NEWS
index 3abc862b0..42cc196d5 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,8 @@ Bug fixes
  - fixed regression when attaching files to a reply
  - wait 20 seconds (instead of 2) before deleting temporary download forms (#2811)
  - avoid raising exceptions when the db is down and we try to access the preferences module (#2813)
+ - we now ignore the SCHEDULE-AGENT property when Thunderbird/Ligthning sends it to avoid
+   not-generating invitation responses for externally received IMIP messages
 
 2.2.5 (2014-06-05)
 ------------------
diff --git a/SoObjects/Appointments/SOGoAppointmentObject.m b/SoObjects/Appointments/SOGoAppointmentObject.m
index 1712ec7ea..0e397928f 100644
--- a/SoObjects/Appointments/SOGoAppointmentObject.m
+++ b/SoObjects/Appointments/SOGoAppointmentObject.m
@@ -1428,9 +1428,11 @@ inRecurrenceExceptionsForEvent: (iCalEvent *) theEvent
 //
 - (BOOL) _shouldScheduleEvent: (iCalPerson *) theOrganizer
 {
+  NSArray *userAgents;
   NSString *v;
   BOOL b;
-  
+  int i;
+ 
   b = YES;
 
   if (theOrganizer && (v = [theOrganizer value: 0  ofAttribute: @"SCHEDULE-AGENT"]))
@@ -1440,6 +1442,27 @@ inRecurrenceExceptionsForEvent: (iCalEvent *) theEvent
 	b = NO;
     }
 
+  //
+  // If we have to deal with Thunderbird/Lightning, we always send invitation
+  // reponses, as Lightning v2.6 (at least this version) sets SCHEDULE-AGENT
+  // to NONE/CLIENT when responding to an external invitation received by
+  // SOGo - so no invitation responses are ever sent by Lightning. See
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=865726 and
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=997784
+  //
+  userAgents = [[context request] headersForKey: @"User-Agent"];
+  
+  for (i = 0; i < [userAgents count]; i++)
+    {
+      if ([[userAgents objectAtIndex: i] rangeOfString: @"Thunderbird"].location != NSNotFound &&
+          [[userAgents objectAtIndex: i] rangeOfString: @"Lightning"].location != NSNotFound)
+        {
+          b = YES;
+          break;
+        }
+    }
+
+
   return b;
 }