How can queued messages be forced to attempt delivery?
Delivery of queued messages can be forced by going to Monitor | Current Status | MTA Status | MTA total by host. Click on the queue in which the messages are located and then click on the "deliver all queued messages" button. This will retry the delivery of those messages. MTA retry time can also be adjusted at the Manage | Network | MTA Configuration page.
How can queued messages be removed/dropped?
In the event of a compromised system, queued messages can be dropped. To do this, go to Manage | Network | MTA Configuration & make note of the existing configuration. Change the Retry interval to 5 minutes and the Bounce after duration to 10 minutes and click Save. Wait 15 minutes and reboot the appliance. Once it is back online, check the MTA Status, it should be clear. Go back to MTA Configuration and reconfigure the Retry interval and Bounce after duration to their original settings.