diff --git a/addons/mail/controllers/thread.py b/addons/mail/controllers/thread.py
index 050fcb16bad..d63ea6751a9 100644
--- a/addons/mail/controllers/thread.py
+++ b/addons/mail/controllers/thread.py
@@ -62,13 +62,7 @@ class ThreadController(http.Controller):
 
     @http.route("/mail/message/post", methods=["POST"], type="json", auth="public")
     def mail_message_post(self, thread_model, thread_id, post_data):
-        if thread_model == "discuss.channel":
-            channel_member_sudo = request.env["discuss.channel.member"]._get_as_sudo_from_request_or_raise(
-                request=request, channel_id=int(thread_id)
-            )
-            thread = channel_member_sudo.channel_id
-        else:
-            thread = request.env[thread_model].browse(int(thread_id)).exists()
+        thread = request.env[thread_model]._get_from_request_or_raise(request, int(thread_id))
         if "body" in post_data:
             post_data["body"] = Markup(post_data["body"])  # contains HTML such as @mentions
         message_data = thread.message_post(
diff --git a/addons/mail/models/discuss/discuss_channel.py b/addons/mail/models/discuss/discuss_channel.py
index 299a5e551a8..ccc62ba00f0 100644
--- a/addons/mail/models/discuss/discuss_channel.py
+++ b/addons/mail/models/discuss/discuss_channel.py
@@ -279,6 +279,13 @@ class Channel(models.Model):
         if not self._cr.fetchone():
             self._cr.execute('CREATE INDEX discuss_channel_member_seen_message_id_idx ON discuss_channel_member (channel_id,partner_id,seen_message_id)')
 
+    @api.model
+    def _get_from_request_or_raise(self, request, thread_id):
+        """Overridden because guests and (portal) users need sudo to post on channels. This ensures they are actually
+        members before granting them access, as well as properly setting up the guest context on the resulting thread
+        if applicable."""
+        return self.env["discuss.channel.member"]._get_as_sudo_from_request_or_raise(request, thread_id).channel_id
+
     # ------------------------------------------------------------
     # MEMBERS MANAGEMENT
     # ------------------------------------------------------------
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index aaf4dc5e57d..730d7db9ecc 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -454,6 +454,10 @@ class MailThread(models.AbstractModel):
         if any(message.message_type != 'comment' for message in messages):
             raise exceptions.UserError(_("Only messages type comment can have their content updated"))
 
+    @api.model
+    def _get_from_request_or_raise(self, request, thread_id):
+        return self.search([("id", "=", thread_id)])
+
     # ------------------------------------------------------------
     # FIELDS HELPERS
     # ------------------------------------------------------------