Delivery mode

Controls when messages send to this mail server are delivered. Possible options are :

• Background
  Messages are delivered as soon as possible in the background.

• Queue only
  Submitted messages are merely placed in the queue, and not delivered until the queue is flushed manually.

• Interactive
  Messages are delivered immediately.

• Deferred
  Like Queue only, but no DNS or NIS lookups are done for submitted messages.

---