diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index 5819b43605a..89859ef5aa2 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -366,7 +366,7 @@ WHERE
 }
 
 function forum_get_email_message_id($postid, $usertoid, $hostname) {
-    return '<moodlepost'.$postid.'to'.$usertoid.'@'.$hostname.'>';
+    return '<'.hash('sha256',$postid.'to'.$usertoid.'@'.$hostname).'>';
 }
 
 /**