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.

  initgroups      is set true if initgroups() is to be called when using the
                  uid and gid set up by the director.

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

  fallback_hosts  fallback host list - relevant only if the director sets up
                  a remote transport for the address.

  errors_address  where to send error messages for this address.

  extra_headers   additional headers to be added to the message for this
                  address.

  remove_headers  the names of headers to be removed from the message for this
                  address

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