The outbox, the destinations and the send: OutboxDispatcherOptions.
One pass over the due deliveries; answers how many were delivered and how many failed. A delivery whose destination
resolve does not give, or rejects for, is marked failed, and the pass goes on. When the outbox has sendClaimed,
each delivery is sent through it, and one it skips (gone, no longer pending, being sent already, or cancelled by a
purge) counts as neither.
Sends what the outbox holds due.