diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -980,7 +980,7 @@
         $mime->headers($headers);
         $mime->setTXTBody("This is a Kolab Groupware object. "
             . "To view this object you will need an email client that understands the Kolab Groupware format. "
-            . "For a list of such email clients please visit http://www.kolab.org/\n\n");
+            . "For a list of such email clients please visit https://www.kolab.org/\n\n");
 
         $ctype = kolab_storage::$version == '2.0' ? $format->CTYPEv2 : $format->CTYPE;
         // Convert new lines to \r\n, to wrokaround "NO Message contains bare newlines"