I belive unix mail files use some other locking scheme than flock or fcntl, it's something with a separate lockfile, but I'm not sure how it works.

Procmail has a utility called lockfile whose manpage might tell you something about this.

In reply to Re: flock on postfix /var/spool/mail/user by ambrus
in thread flock on postfix /var/spool/mail/user by abachus

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