+.new
+In the fourth case,
+the value given for this option must be an absolute path for an
+existing directory.
+The expansion of either the &%directory%& or &%file%&
+option is checked for being a strict (possibly potential) descendant,
+in the filesystem, of the value given.
+.cindex "tainted data" "de-tainting"
+If the check passes then the path checked becomes de-tainted.
+If the check fails then the transport returns failure.
+.wen
+