ROUTERS:

The yield of a router is one of:

  OK        the address was routed and added to addr_local or addr_remote
  DECLINE   the address was not routed; pass to next router, unless no_more
              is set
  PASS      the address has been modified; pass to next router even if no_more
              is set
  FORCEFAIL the address was not routed; do not pass to subsequent routers
  ISLOCAL   the address turned out to be local after all
  DEFER     retry this address later
  ERROR     there was some bad error while routing this address

PASS has the same effect as FAIL, but it can be logged differently. When
routing succeeds, the following field in the address is set:

  transport       points to the transport instance control block; this can
                  be a remote transport or a local transport - the latter is
                  usually a "batched SMTP" transport for passing remote
                  deliveries to some other means of transportation.

  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.

  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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