An existing Symantec Messaging Gateway (SMG) hardware appliance cannot get the update to version 10.6.5-1 or later.
SMG version 10.6.5-1 does not allow hardware appliances that do not have an active, original warranty from Dell to get the new software.
As of 01 June 2018, there are three options:
If you need to download the VM ISO for SMG, please visit Symantec FileConnect.
If you need another copy of your existing license file (SLF), please visit Symantec Licensing and log in to your account.
For more details, visit the Release Notes for SMG 10.6.5.