diff --git a/addons/mail/controllers/thread.py b/addons/mail/controllers/thread.py
index 1227a2c3de1..02024f31219 100644
--- a/addons/mail/controllers/thread.py
+++ b/addons/mail/controllers/thread.py
@@ -97,6 +97,6 @@ class ThreadController(http.Controller):
             raise NotFound()
         if not message_sudo.model or not message_sudo.res_id:
             raise NotFound()
-        request.env[message_sudo.model].browse([message_sudo.res_id])._message_update_content(
+        guest.env[message_sudo.model].browse([message_sudo.res_id])._message_update_content(
             message_sudo, body, attachment_ids=attachment_ids, partner_ids=partner_ids
         )