linked nodes: UseOutboxes (default: true)

BinkIT will continue to outboxes even when this option is set to false, but
SBBSecho won't place any mail files in the outboxes when this option is set to
true.

For PSI-Jack who was surprised that SBBSecho put mail files into outboxes.
Since it appears BinkD supports both outboxes and normal outbound directories
for linked nodes and BinkIT does as well, this shouldn't really make any
difference - just a sysop preference.