An email bounces back to the sender as undeliverable. The Message Audit Log (MAL) in Symantec Messaging Gateway (SMG) display the status "554 5.4.7 [internal] exceeded max time without delivery".
2018 Jan 25 10:22:48 CET (info) ecelerity:  ML-PERMFAIL: Message ID: 4B/28-55573-2F27B5B4, Audit ID 50bbc0e0a-b7cbdae000007ed9-6d-4b5b72f14805, Permanent delivery failure, sender: [email protected], Detail: 554 5.4.7 [internal] exceeded max time without delivery
Undeliverable messages are queued and redelivery is attempted up to 5 days. Undeliverable mail after this period is rejected from the queue, and a notification is sent to the sender.
CAUTION: The default value for how long undeliverable messages are queued is an industry best practice. Symantec does not recommend modifying this value unless absolutely necessary.