diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index efa1897f9f0..0dd89e1c421 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -747,12 +747,6 @@ function forum_cron() {
                     'Auto-Submitted: auto-generated',
                 );
 
-                if ($post->parent) {
-                    // This post is a reply, so add headers for threading (see MDL-22551).
-                    $userfrom->customheaders[] = 'In-Reply-To: ' . forum_get_email_message_id($post->parent, $userto->id, $hostname);
-                    $userfrom->customheaders[] = 'References: '  . forum_get_email_message_id($post->parent, $userto->id, $hostname);
-                }
-
                 $shortname = format_string($course->shortname, true, array('context' => context_course::instance($course->id)));
 
                 // Generate a reply-to address from using the Inbound Message handler.
@@ -772,6 +766,30 @@ function forum_cron() {
                 $posthtml = forum_make_mail_html($course, $cm, $forum, $discussion, $post, $userfrom, $userto,
                         $replyaddress);
 
+                $rootid = forum_get_email_message_id($discussion->firstpost, $userto->id, $hostname);
+
+                if ($post->parent) {
+                    // This post is a reply, so add reply header (RFC 2822).
+                    $parentid = forum_get_email_message_id($post->parent, $userto->id, $hostname);
+                    $userfrom->customheaders[] = "In-Reply-To: $parentid";
+
+                    // If the post is deeply nested we also reference the parent message id and
+                    // the root message id (if different) to aid threading when parts of the email
+                    // conversation have been deleted (RFC1036).
+                    if ($post->parent != $discussion->firstpost) {
+                        $userfrom->customheaders[] = "References: $rootid $parentid";
+                    } else {
+                        $userfrom->customheaders[] = "References: $parentid";
+                    }
+                }
+
+                // MS Outlook / Office uses poorly documented and non standard headers, including
+                // Thread-Topic which overrides the Subject and shouldn't contain Re: or Fwd: etc.
+                $a->subject = $discussion->name;
+                $postsubject = html_to_text(get_string('postmailsubject', 'forum', $a), 0);
+                $userfrom->customheaders[] = "Thread-Topic: $postsubject";
+                $userfrom->customheaders[] = "Thread-Index: " . substr($rootid, 1, 28);
+
                 // Send the post now!
                 mtrace('Sending ', '');