Microsoft Exchange Server receives and processes result information indicating the following:
If a message cannot be sent through its chosen connector, that is, the transfer-out fails, the message is rerouted. If delivery fails, an attempt is made to reassign the message, that is, alternate recipient handling occurs.
If the message is not rerouted or reassigned, checks are made to determine whether a report is required for the recipients. The report is sent only after all responsible recipients have been delivered or transferred successfully, or if the MTA is unable to deliver a subset of recipients for some reason. The one exception is that a report is sent immediately for any completely invalid recipients that cannot be routed at all.