By default, mailagent proceeds with a fixed locking scheme (.lock extension)
plus flock() if asked to do so at Configure time. However, mailagent provides
support for NFS-secure locks and also can use non-standard locking procedures,
configurable from within ~/.mailagent (variables "nfslock" and "mboxlock").

However, it cannot support locking on a rule basis (yet!). The author is
willing to raise the priority of that item if one comes up with a legitimate
need for that feature that could not be worked-around by a PERL escape. ;-)