You have a planned maintenance cycle on your mail server where you will have to take it off the network for a number of hours. You require information on Symantec Messaging Gateway's (SMG) message delivery behavior when messages fail to deliver.

SMG will queue messages in the delivery queue for up to five days in the event that it cannot reach the destination mail server. While the destination servers are unavailable, queued messages will generate a delivery status notification addressed to the sender every four hours while the message is queued. Delivery attempts are initially made at fifteen minutes with an exponentially decreasing backoff. Once connectivity is reestablished, queued messages will be delivered to the destination mail server.

Minimum retry interval for queued messages: 15 minutes
Maximum queue lifetime: 5 days
Delay notification frequency: 4 hours
Maximum bounce message lifetime: 1 day

(NOTE: These are defaults and can be changed in the SMTP Advanced settings)

For more information about the delivery queue schedule, Messaging gateway delivery retry and DSN schedule