Never used the Veeam sandbox feature. Though it uses a virtual lab like their SureBackups, so you might want to check the configuration on that. 

For testing SM upgrades I just do a full VM restore as it's a good excuse to verify the RTO of such a critical system. Just make sure to either put the VM's primary NIC in an isolated subnet before booting it and changing the IP or disable the primary NIC and add one with a different IP for upgrade testing.

Thanks for the reply. I am actually performing a full VM restore into a sandbox ESXi server with our DC's already restored and running (Not using VEEAM sandbox or SureBackup). Everything boots as normal, etc. I assign the correct IP and network information verified with the information in IIS. But so far, unable to connect to the web portal at all. I've tested running updates but am unable to verify settings without the web portal. Is there any sort of configuration file I can double-check or troubleshooting tips?

When you try connecting to the web interface on the restored sandbox server using the http://localhost:9998 binding - what do you get back in the browser? Is there anything being logged either in Event Viewer>Application/System logs or the SmarterMail logs themselves? The SmarterMail logs will be found in C:\SmarterMail\Logs by default.