Update u_e-xmpp-enable.md

docs/u_e-xmpp-enable.md

@@ -2,6 +2,35 @@ XMPP is provided by ejabberd, which describes itself as robust, scalable and ext
 
 So first of all, thanks to ejabberd and its contributers!
 
+## FAQs
+
+- **Are messages stored on the server?**
+
+Not by default. The default setting is to disable the message archive via mod_mam but allow users to enable the function if they want to:
+
+```
+  mod_mam:
+    clear_archive_on_room_destroy: true
+    default: never
+    compress_xml: true
+    request_activates_archiving: true
+```
+
+- **Are uploaded files stored on the server?**
+
+Yes, uploaded files are stored in the volume `xmpp-uploads-vol-1`.
+
+The retention policy saves them for 30 days:
+
+```
+  mod_http_upload_quota:
+    max_days: 30
+```
+
+- **Are messages stored when a JID is offline?**
+
+Yes, up to 1000 messages are stored.
+
 ## Enable XMPP in mailcow
 
 To enable XMPP for a domain, you need to edit the given domain in mailcow UI: