Occasionally, messages cannot be delivered to the recipient that the caller intended to reach. The system behavior in this case depends on the type of sender and the reason that the message could not be delivered.

In general, if Connection cannot deliver the message because of issues that are not likely to be resolved (for example, the caller was disconnected before addressing the message, or the recipient mailbox has been deleted), the message is sent to the Undeliverable Messages distribution list, and Connection sends a nondelivery receipt (NDR) to the sender.

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