A message directed to a local recipient can be enqueued on a "wrong" Server, i.e. on a Server that cannot open the target Account and deliver the message to that Account.
This situation can happen when a message is enqueued on a Frontend Server (Frontend Servers cannot directly open any Account in Shared Domains), or a message is enqueued on a Backend Server that either does not host the target Account (in a Static Cluster), or it cannot open it, because the Account is opened by some other Backend Server (in a Dynamic Cluster).
To solve the problem, the Local Delivery module uses a Delivery channel connection to the proper Backend Server and passes the message there. The receiving Backend immediately opens the target Account, applies its Account-Level Rule and stores the transferred message. This Backend does not enqueue the message.
If there is a temporary problem or a delivery failure, the receiving Backend Server reports the error back, and the message is either delayed in Queue or it is removed from the Queue (with error report messages generated).