Integration tests for RemoteDelivery

Currently tested:

Gateway is working
Direct resolution is working
DNS failures generate a bounce. When bounce processor is specified, and when it is not.

We would need to test :

• That an unreachable MX IP will generate a failover to the second MX when direct resolution
• That an unreachable gateway IP will generate a failover to the second gateway
• If all MX/gateway IPs are not reachable, a bounce is sent
• 5XX Errors don't trigger failover and a bounce is directly sent
• 4XX codes don't trigger failover, and will be retried. Moreover they can succeed on second try, generating no bounce.
• 4XX codes, if answered on each try, will lead to a bounce
  Partial failure (some recipients are failing, some success):
• Only failed (but valid) recipients should be retried (EG 450) and can suddeed on second try
• Only failed (but valid) recipients should be retried (EG 450) and will generate a bounce if repeated
• Invalid recipients (EG 550) directly generate a bounce, and will not be retried
• Valid, and well delivered recipients wont be re-delivered