diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index c340fcc7..66588975 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -39,6 +39,8 @@ abstract class kolab_format
     protected $xmldata;
     protected $loaded = false;
 
+    const VERSION = '3.0';
+
     /**
      * Factory method to instantiate a kolab_format object of the given type
      *
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index 22ddfdb1..ab5bfac5 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -652,6 +652,7 @@ class kolab_storage_folder
         }
         $headers['Date'] = date('r');
         $headers['X-Kolab-Type'] = self::KTYPE_PREFIX . $type;
+        $headers['X-Kolab-Mime-Version'] = kolab_format::VERSION;
         $headers['Subject'] = $object['uid'];
 //        $headers['Message-ID'] = $rcmail->gen_message_id();
         $headers['User-Agent'] = $rcmail->config->get('useragent');