Those /tmp lock files are protected 666, is that really right?
Yes. Shared mailboxes won’t work otherwise. Also, you get into accidental denial of service problems with old lock files left lying around; this happens fairly frequently. The deliberate mischief that can be caused by fiddling with the lock files is small-scale; harassment level at most. There are many — and much more effective — other ways of harassing another user on UNIX. It’s usually not difficult to determine the culprit. Before worrying about deliberate mischief, worry first about things happening by accident!
