diff --git a/lib/kolab_sync_data_email.php b/lib/kolab_sync_data_email.php
--- a/lib/kolab_sync_data_email.php
+++ b/lib/kolab_sync_data_email.php
@@ -455,6 +455,10 @@
                 $meeting['meetingMessageType'] = Syncroton_Model_EmailMeetingRequest::MESSAGE_TYPE_NORMAL;
             }
 
+            // New time proposals aren't supported by Kolab.
+            // This disables the UI elements related to this on the client side
+            $meeting['disallowNewTimeProposal'] = 1;
+
             $result['meetingRequest'] = new Syncroton_Model_EmailMeetingRequest($meeting);
         }