DIRECTORS:

On entry, the "local_part" field of the address points to the local part
we are dealing with, with any prefix and/or suffix removed.

The "domain" field points to the local domain; this has been checked for
applicability to this director. The "require_files" option has not been
checked; this is to allow for the inclusion of $home in the file names for
those directors for which it is relevant.

The yield of a director is one of:

  OK           the address was directed and either added to one of the
               addr_local or addr_remote chains, or one or more new addresses
               were added to addr_new. Note that a local address is permitted
               to specify a remote transport.

  OK_CONTINUE  a new address was added to addr_new as a routing ploy rather
               than as an aliasing or forwarding operation. This means that
               verification should always continue with the new address.

  FAIL         the address was not directed; pass to next director. It is
               permitted for additional addresses to have been added to
               addr_new (or indeed for addresses to have been put on the
               other chains).

  FORCEFAIL    the address was not directed; do not pass to any subseqent
               directors.

  DEFER        retry this address later.

  ERROR        there was some bad error while directing this address (usually a
               problem with the director's configuration).

Both ERROR and DEFER cause the message to be kept on the queue; either may
request freezing.

When direction suceeds, the following fields in the address may be set:

  transport    points to the transport instance control block; this can
               be a local or a remote transport.

  uid, gid     are the uid and gid under which a local transport is to be run
               if the transport does not itself specify them.

  home_dir     the "home" & current directory for transporting, if the transport
               does not specify one itself.

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