diff --git a/mod/forum/classes/output/forum_post.php b/mod/forum/classes/output/forum_post.php
index 431dad61b2e..294b78d0eb0 100644
--- a/mod/forum/classes/output/forum_post.php
+++ b/mod/forum/classes/output/forum_post.php
@@ -378,6 +378,26 @@ class forum_post implements \renderable {
         return 'p' . $this->post->id;
     }
 
+    /**
+     * ID number of the course that the forum is in.
+     *
+     * @return string
+     */
+    public function get_courseidnumber() {
+        return s($this->course->idnumber);
+    }
+
+    /**
+     * The full name of the course that the forum is in.
+     *
+     * @return string
+     */
+    public function get_coursefullname() {
+        return format_string($this->course->fullname, true, array(
+            'context' => \context_course::instance($this->course->id),
+        ));
+    }
+
     /**
      * The name of the course that the forum is in.
      *
diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index 86bb7fed301..ecf08eb464c 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -804,10 +804,16 @@ function forum_cron() {
                     $data->viewfullnames = $userto->viewfullnames[$forum->id];
                 }
 
+                // Not all of these variables are used in the default language
+                // string but are made available to support custom subjects.
                 $a = new stdClass();
-                $a->courseshortname = $data->get_coursename();
-                $a->forumname = $cleanforumname;
                 $a->subject = $data->get_subject();
+                $a->forumname = $cleanforumname;
+                $a->sitefullname = format_string($site->fullname);
+                $a->siteshortname = format_string($site->shortname);
+                $a->courseidnumber = $data->get_courseidnumber();
+                $a->coursefullname = $data->get_coursefullname();
+                $a->courseshortname = $data->get_coursename();
                 $postsubject = html_to_text(get_string('postmailsubject', 'forum', $a), 0);
 
                 // Send the post now!