The steps below are taken from here:
https://www.cisco.com/c/en/us/td/docs/security/security_management/sma/sma14-0/b_sma_admin_guide_14_0/b_NGSMA_Admin_Guide_chapter_01011.html#task_1616749Basically you just need to suspend the listener, and let it sit for a minute to drain the queue... on an SMA that all happens automatically if you use the gui.
Step 1
Take steps to prevent or minimize data loss:
Make sure the new appliance has sufficient disk capacity and the same or greater size allocations for each data type that will be transferred. See About Disk Space Maximums and Allocations.
If you have received any disk space warnings, resolve any disk space issues before upgrading.
Step 2
Save the XML configuration file off the appliance. See caveats at Saving and Exporting the Current Configuration File.
If you need to revert to the pre-upgrade release for any reason, you will need this file.
Step 3
If you are using the Safelist/Blocklist feature, export the list off the appliance.
Click Management Appliance > System Administration > Configuration File and scroll down.
Step 4
Suspend the listeners using the suspendlistener command when running the upgrade from the CLI. If you perform the upgrade from the GUI, listener suspension occurs automatically.
Step 5
Drain the mail queue and the delivery queue.
Step 6
Verify that the upgrade settings are configured as you want them. See Configuring Upgrade and Service Update Settings.