Approved messages from Incidient folder on Standalone quarantine cannot be delivered.
Ensure that first scanner from your SMG in inventory is allowed to send emails using port 25. When messages are approved from Standalone Quarantine routing is different than for the regular spam - message will be routed from Standalone to the first scanner in your SMG inventory. Assuming that this inbound Scanner port 25 can be blocked for outbound SMTP traffic.
Inspect the queues on the problematic scanner to narrow down the delivery issues by following the part using GUI - Status/SMTP/Message Queus/ and select the problematic scanner. In case port 25 is open to the end domain and issue persists contanct Broadcom Support.