fcntl/flock() locking of mailboxes is actually disabled
due to a typo:

--- mailbox.py.orig
+++ mailbox.py
@@ -15,7 +15,7 @@
 import rfc822
 import StringIO
 try:
-    import fnctl
+    import fcntl
 except ImportError:
     fcntl = None

However, once enabled, it doesn't work on FreeBSD - the
lock() method always raises ExternalClashError (tested
on 5.3).  This is because flock(), lockf() and fcntl
locking share the same underlying mechanism on this
system (and probably others), and so the second lock
can never be acquired.