#9559: Append data to single-file mailbox files if messages are only added

If messages were only added, a new file is no longer created and
renamed over the old file when flush() is called on an mbox, MMDF or
Babyl mailbox.
diff --git a/Misc/NEWS b/Misc/NEWS
index 62968d2..aa41678 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -75,6 +75,10 @@
 Library
 -------
 
+- Issue #9559: If messages were only added, a new file is no longer
+  created and renamed over the old file when flush() is called on an
+  mbox, MMDF or Babyl mailbox.
+
 - Issue #14653: email.utils.mktime_tz() no longer relies on system
   mktime() when timezone offest is supplied.