ROUTERS:

The yield of a router is one of:

  OK        the address was routed and added to addr_local or addr_remote
  FAIL      the address was not routed; pass to next router
  FORCEFAIL the address was not routed; do not pass to subsequent routers
  ISLOCAL   the address turned out to be local after all
  PASS      the address has been modified; pass to next router
  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.
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