diff --git a/NEWS b/NEWS
index a2f77a2df..7f500f1cf 100644
--- a/NEWS
+++ b/NEWS
@@ -28,6 +28,7 @@ Enhancements
  - improved confirmation dialog box when deleting events and tasks
  - moved the DN cache to SOGoCache - avoiding sogod restarts after RDN operations
  - don't use the HTML editor with Internet Explorer 7
+ - add message-id header to appointment notifications (#2535)
 
 Bug fixes
  - don't load 'background' attribute (#2437)
diff --git a/SoObjects/Appointments/SOGoCalendarComponent.m b/SoObjects/Appointments/SOGoCalendarComponent.m
index 57f957473..428301564 100644
--- a/SoObjects/Appointments/SOGoCalendarComponent.m
+++ b/SoObjects/Appointments/SOGoCalendarComponent.m
@@ -24,6 +24,7 @@
 
 #import <Foundation/NSDictionary.h>
 #import <Foundation/NSEnumerator.h>
+#import <Foundation/NSProcessInfo.h>
 #import <Foundation/NSString.h>
 #import <Foundation/NSValue.h>
 
@@ -62,6 +63,7 @@
 #import <SOGo/SOGoWebDAVAclManager.h>
 #import <SOGo/WORequest+SOGo.h>
 #import <Appointments/SOGoAppointmentFolder.h>
+#import <Mailer/NSString+Mail.h>
 
 #import "SOGoAptMailICalReply.h"
 #import "SOGoAptMailNotification.h"
@@ -832,6 +834,7 @@
 		  mailDate = [[NSCalendarDate date] rfc822DateString];
 		  [headerMap setObject: mailDate forKey: @"date"];
 		  [headerMap setObject: subject forKey: @"subject"];
+                  [headerMap setObject: [NSString generateMessageID] forKey: @"message-id"];
                   if ([msgType length] > 0)
                     [headerMap setObject: msgType forKey: @"x-sogo-message-type"];
 		  msg = [NGMimeMessage messageWithHeader: headerMap];
@@ -861,8 +864,7 @@
 		           sendMimePart: msg
                            toRecipients: [NSArray arrayWithObject: email]
                                  sender: shortSenderEmail
-                      withAuthenticator: [self
-                                           authenticatorInContext: context]
+                      withAuthenticator: [self authenticatorInContext: context]
                               inContext: context];
 		}
 	    }
@@ -916,6 +918,7 @@
       [headerMap setObject: mailDate forKey: @"date"];
       [headerMap setObject: [[p getSubject] asQPSubjectString: @"UTF-8"]
                     forKey: @"subject"];
+      [headerMap setObject: [NSString generateMessageID] forKey: @"message-id"];
       [headerMap setObject: @"1.0" forKey: @"MIME-Version"];
       [headerMap setObject: @"multipart/mixed" forKey: @"content-type"];
       [headerMap setObject: @"calendar:invitation-reply" forKey: @"x-sogo-message-type"];
@@ -1038,6 +1041,7 @@
   mailDate = [[NSCalendarDate date] rfc822DateString];
   [headerMap setObject: mailDate forKey: @"date"];
   [headerMap setObject: [page getSubject] forKey: @"subject"];
+  [headerMap setObject: [NSString generateMessageID] forKey: @"message-id"];
   [headerMap setObject: @"1.0" forKey: @"MIME-Version"];
   [headerMap setObject: @"text/html; charset=utf-8"
 		forKey: @"content-type"];
diff --git a/SoObjects/Mailer/NSString+Mail.h b/SoObjects/Mailer/NSString+Mail.h
index 5d04fc01b..a4851855e 100644
--- a/SoObjects/Mailer/NSString+Mail.h
+++ b/SoObjects/Mailer/NSString+Mail.h
@@ -1,6 +1,6 @@
 /* NSString+Mail.h - this file is part of SOGo
  *
- * Copyright (C) 2007-2013 Inverse inc.
+ * Copyright (C) 2007-2014 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
@@ -26,6 +26,7 @@
 
 @interface NSString (SOGoExtension)
 
++ (NSString *) generateMessageID;
 - (NSString *) htmlToText;
 - (NSString *) htmlByExtractingImages: (NSMutableArray *) theImages;
 - (NSString *) stringByConvertingCRLNToHTML;
diff --git a/SoObjects/Mailer/NSString+Mail.m b/SoObjects/Mailer/NSString+Mail.m
index cc8164b1c..7e7f1fbaf 100644
--- a/SoObjects/Mailer/NSString+Mail.m
+++ b/SoObjects/Mailer/NSString+Mail.m
@@ -20,8 +20,9 @@
 
 #import <Foundation/NSArray.h>
 #import <Foundation/NSDictionary.h>
-#import <Foundation/NSObject.h>
 #import <Foundation/NSException.h>
+#import <Foundation/NSObject.h>
+#import <Foundation/NSProcessInfo.h>
 #import <Foundation/NSValue.h>
 
 #import <SaxObjC/SaxAttributes.h>
@@ -516,6 +517,19 @@
 
 @implementation NSString (SOGoExtension)
 
++ (NSString *) generateMessageID
+{
+  NSMutableString *messageID;
+  NSString *pGUID;
+
+  messageID = [NSMutableString string];
+  [messageID appendFormat: @"<%@", [SOGoObject globallyUniqueObjectId]];
+  pGUID = [[NSProcessInfo processInfo] globallyUniqueString];
+  [messageID appendFormat: @"@%u>", [pGUID hash]];
+
+  return [messageID lowercaseString];
+}
+
 - (NSString *) htmlToText
 {
   _SOGoHTMLContentHandler *handler;
diff --git a/SoObjects/Mailer/SOGoDraftObject.m b/SoObjects/Mailer/SOGoDraftObject.m
index 5074294dd..bd5a0d469 100644
--- a/SoObjects/Mailer/SOGoDraftObject.m
+++ b/SoObjects/Mailer/SOGoDraftObject.m
@@ -256,19 +256,6 @@ static NSString    *userAgent      = nil;
 
 /* contents */
 
-- (NSString *) _generateMessageID
-{
-  NSMutableString *messageID;
-  NSString *pGUID;
-
-  messageID = [NSMutableString string];
-  [messageID appendFormat: @"<%@", [self globallyUniqueObjectId]];
-  pGUID = [[NSProcessInfo processInfo] globallyUniqueString];
-  [messageID appendFormat: @"@%u>", [pGUID hash]];
-
-  return [messageID lowercaseString];
-}
-
 - (void) setHeaders: (NSDictionary *) newHeaders
 {
   id headerValue;
@@ -288,7 +275,7 @@ static NSString    *userAgent      = nil;
   messageID = [headers objectForKey: @"message-id"];
   if (!messageID)
     {
-      messageID = [self _generateMessageID];
+      messageID = [NSString generateMessageID];
       [headers setObject: messageID forKey: @"message-id"];
     }