diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index a1e90692680..5aec1baf397 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -826,8 +826,8 @@ function forum_cron() {
                 // 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";
+                $threadtopic = html_to_text(get_string('postmailsubject', 'forum', $a), 0);
+                $userfrom->customheaders[] = "Thread-Topic: $threadtopic";
                 $userfrom->customheaders[] = "Thread-Index: " . substr($rootid, 1, 28);
 
                 // Send the post now!
diff --git a/mod/forum/tests/mail_test.php b/mod/forum/tests/mail_test.php
index d979a4f271a..40f5be489fb 100644
--- a/mod/forum/tests/mail_test.php
+++ b/mod/forum/tests/mail_test.php
@@ -184,6 +184,8 @@ class mod_forum_mail_testcase extends advanced_testcase {
         // Add a post to the discussion.
         $record = new stdClass();
         $record->course = $forum->course;
+        $strre = get_string('re', 'forum');
+        $record->subject = $strre . ' ' . $discussion->subject;
         $record->userid = $author->id;
         $record->forum = $forum->id;
         $record->discussion = $discussion->id;
@@ -834,6 +836,33 @@ class mod_forum_mail_testcase extends advanced_testcase {
         $this->assertEquals($expectedsubject, $message->subject);
     }
 
+    /**
+     * Test inital email and reply email subjects
+     */
+    public function test_subjects() {
+        $this->resetAfterTest(true);
+
+        $course = $this->getDataGenerator()->create_course();
+
+        $options = array('course' => $course->id, 'forcesubscribe' => FORUM_FORCESUBSCRIBE);
+        $forum = $this->getDataGenerator()->create_module('forum', $options);
+
+        list($author) = $this->helper_create_users($course, 1);
+        list($commenter) = $this->helper_create_users($course, 1);
+
+        $strre = get_string('re', 'forum');
+
+        // New posts should not have Re: in the subject.
+        list($discussion, $post) = $this->helper_post_to_forum($forum, $author);
+        $messages = $this->helper_run_cron_check_count($post, 2);
+        $this->assertNotContains($strre, $messages[0]->subject);
+
+        // Replies should have Re: in the subject.
+        $reply = $this->helper_post_to_discussion($forum, $discussion, $commenter);
+        $messages = $this->helper_run_cron_check_count($reply, 2);
+        $this->assertContains($strre, $messages[0]->subject);
+    }
+
     /**
      * dataProvider for test_forum_post_email_templates().
      */