diff --git a/offlineimap.conf b/offlineimap.conf
index 05bf711..cdf79d0 100644
--- a/offlineimap.conf
+++ b/offlineimap.conf
@@ -1223,6 +1223,11 @@ remoteuser = username
 # e.g. backup an IMAP server. The readonly setting can be applied to any
 # type of Repository (Maildir, Imap, etc).
 #
+# Notice that this does NOT mean that the local Maildir can be used with a
+# read-only filesystem When OfflineImap uploads a new email, it is assigned a
+# new UID by the remote which is stored in the filename. Otherwise, Offlineimap
+# would have no way to map the UID to the file.
+#
 #readonly = False