it should be noted that sendmail (all versions, to my knowledge)
ignores flock.
just blatently ignores it if you flock an mbox.
however, it does recognize its own locking system, which
consists of a mboxname.lock file in the same directory
as the mbox.
this does create problems with write access and such, and
is just a horrible workaround in my opinion, however
will work fine.