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Here's how Safety Net is similar to the transport dumpster in Exchange:
Safety Net is a queue that's associated with the Transport service on a Mailbox server. This queue stores copies of messages that were successfully processed by the server.
You can specify how long Safety Net stores copies of the successfully processed messages before they expire and are automatically deleted. The default is2 days.
Here's how Safety Net is different in Exchange:
Safety Net doesn't require DAGs. For Mailbox servers that don't belong to a DAGs, Safety Net stores copies of the delivered messages on other Mailbox servers in the local Active Directory site.
Safety Net itself is now redundant, and is no longer a single point of failure. This introduces the concept of the Primary Safety Net and the Shadow Safety Net. If the Primary Safety Net is unavailable for more than hours, resubmit requests become shadow resubmit requests, and messages are re-delivered from the Shadow Safety Net.
Safety Net takes over some responsibility from shadow redundancy in DAG environments. Shadow redundancy doesn't need to keep another copy of the delivered message in a shadow queue while it waits for the delivered message to replicate to the passive copies of mailbox database on the other Mailbox servers in the DAG. The copy of the delivered message is already stored in Safety Net, so the message can be resubmitted from Safety Net if necessary.
In Exchange, transport high availability is more than just a best effort for message redundancy. Exchange attempts to guarantee message redundancy. Because of this, you can't specify a maximum size limit for Safety Net. You can only specify how long Safety Net stores messages before they're automatically deleted.
